Science  for  Everybody 


ASTRONOMY  FOR 
EVERYBODY 

A  Popular  Exposition  of  the  Wonders 
of  the  Heavens 

BY 

SIMON   NEWCOMB,    LL.D. 

\  \ 

Professor,  U.  &  N.,  retired 


FULLY   ILLUSTRATED 


GARDEN  CITY         NEW  YORK 

DOUBLEDAY,  PAGE  &  COMPANY 

1913 


Copyright,  1902,  by 
DOUBLEDAY,  PAGE  &  COMPANY 


Total  Eclipse  of  the  Sun  of  May  29,  1900. 
Photographed  by  the  party  of  the  Smithsonian  Institution. 


Preface 


THE  present  work  grew  out  of  articles  contributed  to 
McClure's  Magazine  a  few  years  since  on  the  Unsolved 
Problems  of  Astronomy,  Total  Eclipses  of  the  Sun,  and 
other  subjects.  The  interest  shown  in  these  articles 
suggested  an  exposition  of  the  main  facts  of  astronomy 
in  the  same  style.  The  result  of  the  attempt  is  now 
submitted  to  the  courteous  consideration  of  the  reader. 

The  writer  who  attempts  to  set  forth  the  facts  of  as- 
tronomy without  any  use  of  technical  language  finds  him- 
self in  the  dilemma  of  being  obliged  either  to  convey 
only  a  very  imperfect  idea  of  the  subject,  or  to  enter 
upon  explanations  of  force  and  motion  which  his  reader 
ma3'  find  tedious.  In  grappling  with  this  difficulty  the 
author  has  followed  a  middle  course,  trying  to  present 
the  subject  in  such  a  way  as  to  be  intelligible  and  inter- 
esting to  every  reader,  and  entering  into  technical  ex- 
planations only  when  necessary  to  the  clear  understand- 
ing of  such  matters  as  the  measure  of  time,  the  changes 
of  the  seasons,  the  varying  positions  of  the  constella- 
tions, and  the  aspects  of  the  planets.  It  is  hoped  that 
the  reader  who  does  not  wish  to  master  these  subjects 
will  find  enough  to  interest  him  in  the  descriptions  and 
illustrations  of  celestial  scenery  to  which  the  bulk  of  the 
work  is  devoted. 

The  author  is  indebted  to  Mr.  Secretary  Langlev,  of 
the  Smithsonian  Institution,  for  the  use  of  the  picture 
which  forms  the  frontispiece. 

SIMON  NEWCOMB. 

Washington,  October,  1902. 

934478 


Contents 


PART  I.     THE  CELESTIAL  MOTIONS 

PAGE 

L    A  VIEW  OP  THE  UNIVEKSB 3 

What  the  universe  is 6 

A  model  of  the  universe 7 

IL    ASPECTS  OF  THE  HEAVENS 9 

Apparent  daily  revolution  of  the  stars 12 

Changes  in  the  motions  as  we  journey  south 15 

HE.    RELATION  OP  TIME  AND  LONGITUDE 19 

Local  time •. .  21 

Standard  time , 21 

Where  the  day  changes 23 

IV.    How  THE  POSITION  OF  A  HEAVENLY  BODY  is  DEFINED.  ...  25 

^*         Circles  of  the  celestial  sphere 27 

V.    THE  ANNUAL  MOTION  OF  THE  EARTH  AND  ITS  RESULTS  ...  31 

The  sun's  apparent  path  in  the  sky 32 

The  ecliptic 33 

The  equinoxes  and  solstices 37 

The  seasons 39 

Relations  between  real  and  apparent  motions  summed 

up 39 

The  year  and  the  precession  of  the  equinoxes 41 

PART  II.     ASTRONOMICAL  INSTRUMENTS 

L    THE  REFRACTING  TELESCOPE 47 

The  lenses  of  a  telescope 48 

The  image  of  a  distant  object 51 

Power  and  defects  of  a  telescope 53 

Mounting  of  the  telescope 55 

The  making  of  telescopes 59 

Fraunhof  er  and  Alvan  Clark 61 


viii  CONTENTS 

PAGE 

IE.     THE  REFLECTING  TELESCOPE 67 

III.  THE  PHOTOGRAPHIC  TELESCOPE : „ 71 

IY.     THE  SPECTROSCOPE 73 

Nature  and  wave  length  of  light 74 

The  spectrum „ . . .  75 

How  the  stars  are  analysed 76 

Y.     OTHER  ASTRONOMICAL  INSTRUMENTS. 79 

The  meridian  circle  and  clock 80 

How  the  position  of  stars  are  determined 81 

PART  III.     THE  SUN",   EARTH,  AND  MOON 

I.    AN  INTRODUCTORY  GLANCE  AT  THE  SOLAR  SYSTEM 87 

H.     THE  SUN «. „ 91 

General  description  . .  „ 91 

Eotation  of  the  sun . . . . 93 

Density  and  gravity 94 

Spots  on  the  sun 95 

The  Faculae ...... 98 

The  prominences  and  chromosphere 99 

How  the  sun  is  made  up. . . . . . . . . .. 10.1 

The  source  of  the  sun's  heat. . . . .  103 

in.     THE  EARTH 107 

^,  Measuring  the  earth .....»»...*...; 107 

The  earth's  interior  .................. j 108 

Its  gravity  and  density, .  .v.. Ill 

Variations  of  latitude 114 

The  atmosphere „ 116 

IV.  THE  MOON 119 

Distance  of  the  moon „ 120 

Revolution  and  phases. 120 

The  surface  of  the  moon „ 123 

Is  there  air  or  water  on  the  moon  ?  „ 127 

Eotation  of  the  moon 0 128 

How  the  moon  produces  the  tides 129 

V.    ECLIPSES  OF  THE  MOON . .'. 133 

The  nodes  of  the  moon's  orbit. 134 

Eclipse  seasons ....... 134 

How  an  eclipse  of  the  moon  looks » „ 136 


CONTENTS  ix 

PAGE 

TI.    ECLIPSES  OF  THE  SUN 139 

Central,  total,  and  annular  eclipses, , . . .  0 140 

Beauty  of  a  total  eclipse .**.-. 141 

Ancient  eclipses 143 

Prediction  of  eclipses 144 

The  sun's  appendages  ............ ...........  145 

The  corona  ............  „ .  „ „.......„  147 

PART  IV.     THE  PLANETS  AND  THEIR  SATELLITES 

I.     ORBITS  AND  ASPECTS  OF  THE  PLANETS  . . . . . . . .... . ... . .  151 

Distances  of  the  planets = . .  154 

Bode's  law 154 

Kepler's  laws  ........... 155 

II.     THE  PLANET  MERCURY 157 

Surface  and  rotation  of  Mercury 159 

Observations  of  Schroter,  Herschel,  Schiaparelli,  and 

Lowell 160 

Phases  of  Mercury I 161 

Transits  of  Mercury .« . 163 

m.     THE  PLANET  VENUS 167 

The  morning  and  evening  star  . . . .  „ 167 

Eotation  of  Venus ....... 169 

Atmosphere  of  Venus 172 

Has  Venus  a  satellite  ? 174 

Transits  of  Venus 175 

IV.  THE  PLANET  MARS  . , 177 

Distance,  dates  of  opposition,  etc. 178 

Surface  and  rotation  of  Mars. 179 

The  canals  of  Mars 180 

Probable  nature  of  the  channels 184 

The  atmosphere  of  Mars 185 

Supposed  winter  snowfall  near  the  poles 187 

The  satellites  of  Mars 188 

V.  THE  GROUP  OF  MINOR  PLANETS 191 

Discovery  of  Ceres 191 

Hunting  asteroids .<, 192 

Orbits  of  the  asteroids 194 

Groupings  of  the  orbits . .. . 195 

The  most  curious  of  the  asteroids 198 


x  CONTENTS 

PAGE 

VL    JUPITER  AND  ITS  SATELLITES 201 

Aspect  of  Jupiter 201 

Surface 202 

Constitution ...«,.„ 205 

Eotation 206 

Resemblance  of  Jupiter  to  the  sun . „ .  206 

The  satellites  of  Jupiter 208 

VII.    SATURN  AND  ITS  SYSTEM 213 

Aspects  of  Saturn 213 

Satellites  of  Saturn 214 

Varying  aspects  of  Saturn's  rings 215 

What  the  rings  are 219 

System  of  Saturn's  satellites 220 

Physical  constitution  of  Saturn 224 

VIII.    URANUS  AND  ITS  SATELLITES ...„.,.  225 

Discovery  of  Uranus 225 

Old  observations • 226 

Constitution  of  the  planet 227 

Its  satellites 227 

IX.    NEPTUNE  AND  ITS  SATELLITE 231 

History  of  the  discovery  of  Neptune 232 

Satellite  of  Neptune 235 

X.     How  THE  HEAVENS  ARE  MEASURED 237 

Parallax 237 

Measurement  by  the  motion  of  light 239 

Measurement  by  the  sun's  gravitation 240 

Results  of  measurements  of  the  sun's  distance 242 

XI.     GRAVITATION  AND  THE  WEIGHING  OF  THE  PLANETS 243 

Accuracy  of   astronomical  predictions  based  on  the 

theory  of  gravitation 244 

How  the  planets  are  weighed . .  „ . . 246 

PART  V.     COMETS  AND  METEORIC  BODIES 

I.     COMETS 255 

Description  of  a  comet ....... 255 

Orbits  of  comets 257 

Halley's  comet 260 

Comets  which  have  disappeared   262 


CONTENTS  xi 

PAGE 

Encke's  comet 264 

Capture  of  comets  by  Jupiter 265 

Whence  come  comets  ? 266 

Brilliant  comets  of  our  time 267 

Nature  of  comets 274 

II.    METEORIC  BODIES 277 

Meteors 277 

Cause  of  meteors 278 

Meteoric  showers 279 

Connection  of  comets  and  meteors 281 

The  zodiacal  light 283 

The  impulsion  of  light 286 

PART  VI.     THE  FIXED  STARS 

I.    GENERAL  REVIEW 291 

- —  Stars  and  nebulae '293 

-  Spectra  of  the  stars 293 

Density  and  heat  of  the  stars 296 

IL    ASPECT  OF  THE  SKY 299 

The  Milky  Way 299 

Brightness  of  the  stars 300 

Number  of  stars 301 

Colours 303 

Collection  into  constellations 303 

III.  DESCRIPTION  OP  THE  CONSTELLATIONS 305 

To  find  the  sidereal  time 306 

The  northern  constellations . . . .  307 

The  autumnal  constellations 309 

The  winter  constellations 313 

The  spring  constellations 316 

The  summer  constellations 317 

IV.  THE  DISTANCES  OF  THE  STARS „ 321 

V.     THE  MOTIONS  OF  THE  STARS 325 

VL     VARIABLE  AND  COMPOUND  STARS  . ,  329 


List  of  Illustrations 


PAGE 

Total  Eclipse  of  the  Sun  of  May  29,  1900.     Photographed  by  the 

party  of  the  Smithsonian  Institution Frontispiece 

The  Celestial  Sphere  as  it  appears  to  us 13 

The  Northern  Sky  and  the  Pole  Star 16 

Circles  of  the  Celestial  Sphere 27 

The  Sun  Crossing  the  Equator  about  March  Twentieth 32 

The  Orbit  of  the  Earth  and  the  Zodiac. 33 

How  the  Obliquity  of  the  Ecliptic  Produces  the  Changes  of  Seasons  35 
Apparent  Motion  of  the  Sun  along  the  Ecliptic  in  Spring  and 

Summer 36 

Apparent  Motion  of  the  Sun  from  March  till  September 37 

Precession  of  the  Equinoxes 43 

Section  of  the  Object-glass  of  a  Telescope 50 

Axes  on  which  a  Telescope  turns 57 

Great  Telescope  of  the  Yerkes  Observatory 65 

Section  of  a  Newtonian  Eeflecting  Telescope 69 

Wave  Length  of  Light 74 

Arrangement  of  the  Colours  of  the  Spectrum 75 

A  Meridian  Instrument 80 

Appearance  of  a  Sun-spot 96 

Frequency  of  Sun-spots  in  Different  Latitudes  on  the  Sun 97 

Revolution  of  the  Moon  Round  the  Earth 121 

Mountainous  Surface  of  the  Moon 124 

Showing  how  the  Moon  would  Move  if  it  did  not  Rotate  on  its 

Axis 129 

How  the  Moon  Produces  Two  Tides  in  a  Day 131 

The  Moon  in  the  Shadow  of  the  Earth. ..                                          .  133 


xiv  LIST  OF  ILLUSTRATIONS 

PAGE 

Passage  of  the  Moon  through  the  Earth's  Shadow ,    .  136 

The  Shadow  of  the  Moon  Thrown  on  the  Earth  during  a  Total 

Eclipse  of  the  Sun 139 

The  Moon  Passing  Centrally  over  the  Sun  during  an  Annular 

Eclipse 140 

Orbits  of  the  Four  Inner  Planets 152 

Conjunctions  of  Mercury  with  the  Sun 158 

Elongations  of  Mercury 159 

Phases  of  Venus  in  Different  Points  of  its  Orbit 168 

Effect  of  the  Atmosphere  of  Venus  during  the  Transit  of  1882. .  172 
Map  of  Mars  and  its  Canals  as  drawn  at  the  Lowell  Observatory  181 
Drawings  of  Lacus  Solis  on  Mars,  by  Messrs.  Campbell  and 

Hussey 183 

Separation  of  the  Minor  Planets  into  Groups 195 

Distribution  of  the  Orbits  of  the  Minor  Planets 196 

Telescopic  Views  of  Jupiter,  one  with  the  Shadow  of  a  Satellite 

Crossing  the  Planet 204 

Perpendicular  View  of  the  Rings  of  Saturn 216 

Showing  how  the  Direction  of  the  Plane  of  Saturn's  Rings  re- 
mains Unchanged 217 

Disappearance  of  the  Rings  of  Saturn,  according  to  Barnard, 

when  seen  edgewise 218 

Orbits  of  Titan  and  Hyperion,  showing  their  relation , . . .  222 

Measure  of  the  Distance  of  an  Inaccessible  Object  by  Triangula- 

tion 237 

Parabolic  Orbit  of  a  Comet 257 

Donati's  Comet,  as  drawn  by  G.  P.  Bond 268 

Head  of  Donati's  Comet,  drawn  by  G.  P.  Bond 270 

Great  Comet  of  1859,  drawn  by  G.  P.  Bond 272 

The  Zodiacal  Light  in  February  and  March 284 

Ursa  Major,  or  The  Dipper 307 

Ursa  Minor 308 

Cassiopeia 308 

Lyra,  the  Harp 311 


LIST  OF  ILLUSTRATIONS  xv 

PAGE 

The  Hyades 313 

The  Pleiades,  as  seen  with  the  naked  eye 313 

Telescopic  View  of  the  Pleiades,  with  Names  of  the  Brighter  Stars  314 

Orion 316 

The  Northern  Crown 317 

Aquila 318 

Delphinns,  the  Dolphin 318 

The  Great  Cluster  of  Hercules,  photographed  at  the  Lick  Observ- 
atory   319 

Scorpius,  the  Scorpion 320 

Measurement  of  the  Parallax  of  a  Star 322 

Arcturus  and  the  Surrounding  Stars  in  Constellation  Bootes. . . .  328 


PART    I 
THE    CELESTIAL    MOTIONS 


I 

A  VIEW  OF  THE  UNIVERSE 

LET  us  enter  upon  our  subject  by  taking  a  general 
view  of  this  universe  in  which  we  live,  fancying  ourselves 
looking  at  it  from  a  point  without  its  limits.  Far  away, 
indeed,  is  the  point  we  must  choose.  To  give  a  concep- 
tion of  the  distance,  let  us  measure  it  by  the  motion  of 
light.  This  agent,  darting  through  186,000  miles  in 
every  second,  would  make  the  circuit  of  the  earth  several 
times  between  two  ticks  of  a  watch.  The  standpoint 
which  we  choose  will  probably  be  well  situated  if  we  take 
it  at  a  distance  through  which  light  would  travel  in 
100,000  years.  So  far  as  we  know,  we  should  at  this 
point  find  ourselves  in  utter  darkness,  a  black  and  star- 
less sky  surrounding  us  on  all  sides.  But,  in  one  direc- 
tion, we  should  see  a  large  patch  of  feeble  light  spread- 
ing over  a  considerable  part  of  the  heavens  like  a  faint 
cloud  or  the  first  glimmer  of  a  dawn.  Possibly  there 
might  be  other  such  patches  in  different  directions,  but 
of  these  we  know  nothing.  The  one  which  we  have  men- 
tioned, and  which  we  call  the  universe,  is  that  which  we 
are  to  inspect.  We  therefore  fly  toward  it — how  fast  we 
need  not  say.  To  reach  it  in  a  month  we  should  have 
to  go  a  million  times  as  fast  as  light.  As  we  approach, 
it  continually  spreads  out  over  more  of  the  black  sky, 


\=JELESTIAL    MOTIONS 

lengiK  *M*f 'covers,  the  region  behind  us  being 
still  entirely  black. 

Before  reaching  this  stage  we  begin  to  see  points  of 
light  glimmering  here  and  there  in  the  mass.  Continu- 
ing our  course,  these  points  become  more  numerous,  and 
seem  to  move  past  us  and  disappear  behind  us  in  the 
distance,  while  new  ones  continually  come  into  view  in 
front,  as  the  passengers  on  a  railway  train  see  landscape 
and  houses  flit  by  them.  These  are  stars,  which,  when 
we  get  well  in  among  them,  stud  the  whole  heavens  as 
we  see  them  do  at  night.  We  might  pass  through  the 
whole  cloud  at  the  enormous  speed  we  have  fancied,  with- 
out seeing  anything  but  stars  and,  perhaps,  a  few  great 
nebulous  masses  of  foggy  light  scattered  here  and  there 
among  them. 

But  instead  of  doing  this,  let  us  select  one  particular 
star  and  slacken  our  speed  to  make  a  closer  inspection 
of  it.  This  one  is  rather  a  small  star;  but  as  we  ap- 
proach it,  it  seems  to  our  eyes  to  grow  brighter.  In  time 
it  shines  like  Venus.  Then  it  casts  a  shadow;  then  we 
can  read  by  its  light ;  then  it  begins  to  dazzle  our  eyes. 
It  looks  like  a  little  sun.  It  is  the  Sun ! 

Let  us  get  into  a  position  which,  compared  with  the  dis- 
tances we  have  been  travelling,  is  right  alongside  of  the 
sun,  though,  expressed  in  our  ordinary  measure,  it  may 
be  a  thousand  million  miles  away.  Now,  looking  down 
and  around  us,  we  see  eight  star-like  points  scattered 
around  the  sun  at  different  distances.  If  we  watch  them 
long  enough  we  shall  see  them  all  in  motion  around  the 
sun,  completing  their  circuit  in  times  ranging  from  three 


A    VIEW    OF    THE    UNIVERSE  5 

months  to  more  than  160  years.  They  move  at  very 
different  distances ;  the  most  distant  is  seventy  times  as 
far  as  the  nearest. 

These  star-like  bodies  are  the  planets.  By  careful 
examination  we  see  that  they  differ  from  the  stars  in 
being  opaque  bodies,  shining  only  by  light  borrowed 
from  the  sun. 

Let  us  pay  one  of  them  a  visit.  We  select  the  third 
in  order  from  the  sun.  Approaching  it  in  a  direction 
which  we  may  call  from  above,  that  is  to  say  from  a 
direction  at  right  angles  to  the  line  drawn  from  it  to 
the  sun,  we  see  it  grow  larger  and  brighter  as  we  get 
nearer.  When  we  get  very  near,  we  sec  it  looking  like 
a  half-moon — one  hemisphere  being  in  darkness  and  the 
other  illuminated  by  the  sun's  rays.  As  we  approach 
yet  nearer,  the  illuminated  part,  always  growing  larger 
to  our  sight,  assumes  a  mottled  appearance.  Still  ex- 
panding, this  appearance  gradually  resolves  itself  into 
oceans  and  continents,  obscured  over  perhaps  half  their 
surface  by  clouds.  The  surface  upon  which  we  are  look- 
ing continually  spreads  out  before  us,  filling  more  and 
more  of  the  sky,  until  we  see  it  to  be  a  world.  We  land 
upon  it,  and  here  we  are  upon  the  earth. 

Thus,  a  point  which  was  absolutely  invisible  while  we 
were  flying  through  the  celestial  spaces,  which  became  a 
star  when  we  got  near  the  sun,  and  an  opaque  globe  when 
yet  nearer,  now  becomes  the  world  on  which  we  live. 

This  imaginary  flight  makes  known  to  us  a  capital 
fact  of  astronomy:  The  great  mass  of  stars  which  stud 
the  heavens  at  night  are  suns.  To  express  the  idea  in 


6  THE    CELESTIAL    MOTIONS 

another  way,  the  sun  is  merely  one  of  the  stars.  Com- 
pared with  its  fellows  it  is  rather  a  small  one,  for  we 
know  of  stars  that  emit  thousands  or  even  tens  of  thou- 
sands of  times  the  light  and  heat  of  the  sun.  Measur- 
ing things  simply  by  their  intrinsic  importance,  there  is 
nothing  special  to  distinguish  our  sun  from  the  hundreds 
of  millions  of  its  companions.  Its  importance  to  us  and 
its  comparative  greatness  in  our  eyes  arise  simply  from 
the  accident  of  our  relation  to  it. 

The  great  universe  of  stars  which  we  have  described 
looks  to  us  from  the  earth  just  as  it  looked  to  us  during 
our  imaginary  flight  through  it.  The  stars  which  stud 
our  sky  are  the  same  stars  which  we  saw  on  our  flight. 
The  great  difference  between  our  view  of  the  heavens 
and  the  view  from  a  point  in  the  starry  distances  is  the 
prominent  position  occupied  by  the  sun  and  planets. 
The  former  is  so  bright  that  during  the  daytime  it  com- 
pletely obliterates  the  stars.  If  we  could  cut  off  the 
sun's  rays  from  any  very  wide  region,  we  should  see  the 
stars  around  the  sun  in  the  daytime  as  well  as  by  night. 
These  bodies  surround  us  in  all  directions  as  if  the  earth 
were  placed  in  the  centre  of  the  universe,  as  was  sup- 
posed by  the  ancients. 

What  the  Universe  Is 

f> 

We  may  connect  what  we  have  just  learned  about  the 
the  universe  at  large  with  what  we  see  in  the  heavens. 
What  we  call  the  heavenly  bodies  are  of  two  classes. 
One  of  these  comprises  the  millions  of  stars  the  arrange- 
ment and  appearance  of  which  we  have  just  described. 


WHAT    THE    UNIVERSE    IS  7 

The  other  comprises  a  single  star,  which  is  for  us  the 
most  important  of  all,  and  the  bodies  connected  with  it. 
This  collection  of  bodies,  with  the  sun  in  its  centre,  forms 
a  little  colony  all  by  itself,  which  we  call  the  solar  system. 
The  feature  of  this  system  which  I  wish  first  to  impress 
on  the  reader's  mind  is  its  very  small  dimensions  when 
compared  with  the  distances  between  the  stars.  All 
around  it  are  spaces  which,  so  far  as  we  yet  know,  are 
quite  void  through  enormous  distances.  If  we  could  fly 
across  the  whole  breadth  of  the  system,  we  should  not  bo 
able  to  see  that  we  were  any  nearer  the  stars  in  front  of 
us,  nor  would  the  constellations  look  in  any  way  different 
from  what  they  do  from  our  earth.  An  astronomer  armed 
with  the  finest  instruments  would  be  able  to  detect  a 
change  only  by  the  most  exact  observations,  and  then 
only  in  the  case  of  the  nearer  stars. 

A  conception  of  the  respective  magnitudes  and  dis- 
tances of  the  heavenly  bodies,  which  will  help  the  reader 
in  conceiving  of  the  universe  as  it  is,  may  be  gained  by 
supposing  us  to  look  at  a  little  model  of  it.  Let  us 
imagine  that,  in  this  model  of  the  universe,  the  earth  on 
which  we  dwell  is  represented  by  a  grain  of  mustard  seed. 
The  moon  will  then  be  a  particle  about  one  fourth  the 
diameter  of  the  grain,  placed  at  a  distance  of  an  inch 
from  the  earth.  The  sun  will  be  represented  by  a  large 
apple,  pi  ced  at  a  distance  of  forty  feet.  Other  planets, 
ranging  in  size  from  an  invisible  particle  to  a  pea,  must 
be  imagined  at  distances  from  the  sun  varying  from  ten 
feet  to  a  quarter  of  a  mile.  We  must  then  imagine  all 
these  little  objects  to  be  slowly  moving  around  the 


8  THE    CELESTIAL    MOTIONS 

sun  at  their  respective  distances,  in  times  varying  from 
three  months  to  160  years.  As  the  mustard  seed  per- 
forms its  revolution  in  the  course  of  a  year  we  must 
imagine  the  moon  to  accompany  it,  making  a  revolution 
around  it  every  month. 

On  this  scale  a  plan  of  the  whole  solar  system  can  be 
laid  down  in  a  field  half  a  mile  square.  Outside  of  this 
field  we  should  find  a  tract  broader  than  the  whole  con- 
tinent of  America  without  a  visible  object  in  it  unless 
perhaps  comets  scattered  around  its  border.  Far  beyond 
the  limits  of  the  American  continent  we  should  find  the 
nearest  star,  which,  like  our  sun,  might  be  represented 
by  a  large  apple.  At  still  greater  distances,  in  every 
direction,  would  be  other  stars,  but,  in  the  general  aver- 
age, they  would  be  separated  from  each  other  as  widely 
as  the  nearest  star  is  from  the  sun.  A  region  of  the 
little  model  as  large  as  the  whole  earth  might  contain 
only  two  or  three  stars. 

We  see  from  this  how,  in  a  flight  through  the  universe, 
like  the  one  we  have  imagined,  we  might  overlook  such 
an  insignificant  little  body  as  our  earth,  even  if  we  made 
a  careful  search  for  it.  We  should  be  like  a  person  fly- 
ing through  the  Mississippi  Valley,  looking  for  a  grain 
of  mustard  seed  which  he  knew  was  hidden  somewhere 
on  the  American  continent.  Even  the  bright  shining 
apple  representing  the  sun  might  be  overlooked  unless 
we  happened  to  pass  quite  near  it. 


n 

ASPECTS  OF  THE  HEAVENS 

THE  immensity  of  the  distances  which  separate  us 
from  the  heavenly  bodies  makes  it  impossible  for  us  to 
form  a  distinct  conception  of  the  true  scale  of  the  uni- 
verse, and  very  difficult  to  conceive  of  the  heavenly 
bodies  in  their  actual  relations  to  us.  If,  on  looking  at  a 
body  in  the  sky,  there  were  any  way  of  estimating  its 
distance,  and  if  our  eyes  were  so  keen  that  we  could  see 
the  minutest  features  on  the  surface  of  the  planets  and 
stars,  the  true  structure  of  the  universe  would  have  been 
obvious  from  the  time  that  men  began  to  study  the  heav- 
ens. A  little  reflection  will  make  it  obvious  that  if  we 
could  mount  above  the  earth  to  a  distance  of,  say,  ten 
thousand  times  its  diameter,  so  that  it  would  no  longer 
have  any  perceptible  size,  it  would  look  to  us,  in  the  light 
of  the  sun,  like  a  star  in  the  sky.  The  ancients  had  no 
conception  of  distances  like  this,  and  so  supposed  that 
the  heavenly  bodies  were,  as  they  appeared,  of  a  con- 
stitution totally  different  from  that  of  the  earth.  We 
ourselves,  looking  at  the  heavens,  are  unable  to  conceive 
of  the  stars  being  millions  of  times  farther  than  the 
planets.  All  look  as  if  spread  out  on  one  sky  at  the  same 
distance.  We  have  to  learn  their  actual  arrangement 
and  distances  by  reason. 

It  is  from  the  impossibility  of  conceiving  these  enor 


10  THE    CELESTIAL    MOTIONS 

mous  differences  in  the  distances  of  objects  on  the  earth 
and  the  heavens,  that  the  real  difficulty  of  forming  a 
mental  picture  of  them  in  their  true  relation  arises.  I 
shall  ask  the  reader's  careful  attention  in  an  attempt 
to  present  these  relations  in  the  simplest  way,  so  as  to 
connect  things  as  they  are  with  things  as  we  see  them. 

Let  us  suppose  the  earth  taken  away  from  under  our 
feet,  leaving  us  hanging  in  mid  space.  We  should  then 
see  the  heavenly  bodies — sun,  moon,  planets,  and  stars — 
surrounding  us  in  every  direction,  up  and  down,  east  and 
west,  north  and  south.  The  eye  would  rest  on  nothing 
else.  As  we  have  just  explained,  all  these  objects  would 
seem  to  us  to  be  at  the  same  distance. 

A  great  collection  of  points  scattered  in  every  direc- 
tion at  an  equal  distance  from  one  central  point,  must 
all  lie  upon  the  inner  surface  of  a  hollow  sphere.  It  fol- 
lows that,  in  the  case  supposed,  the  heavenly  bodies  will 
appear  to  us  as  if  set  in  a  sphere  in  the  centre  of  which 
we  appear  to  be  placed.  Since  one  of  the  final  objects 
of  astronomy  is  to  learn  the  directions  of  the  heavenly 
bodies  from  us,  this  apparent  sphere  is  talked  about  in 
astronomy  as  if  it  were  a  reality.  It  is  called  the  celes- 
tial sphere.  In  the  case  we  have  supposed,  with  the  earth 
out  of  the  way,  all  the  heavenly  bodies  on  this  sphere 
would  at  any  moment  seem  at  rest.  The  stars  would  re- 
main apparently  at  rest  day  after  day  and  week  after 
week.  It  is  true  that,  by  watching  the  planets,  we  should 
in  a  few  days  or  weeks,  as  the  case  might  be,  see  their 
slow  motion  around  the  sun,  but  this  would  not  be  per- 
ceptible at  once.  Our  first  impression  would  be  that  the 


ASPECTS    OF    THE    HEAVENS  11 

sphere  was  made  of  some  solid,  crystalline  substance, 
and  that  the  heavenly  bodies  were  fastened  to  its  inner 
surface.  The  ancients  had  this  notion,  which  they 
brought  yet  nearer  the  truth  by  fancying  a  number  of 
these  spheres  fitting  inside  of  each  other  to  represent  the 
different  distances  of  the  heavenly  bodies. 

With  this  conception  well  in  mind,  let  us  bring  the 
earth  back  under  our  feet.  Now  we  have  to  make  a  draft 
upon  the  reader's  power  of  conception.  Considered  in 
its  relation  to  the  magnitude  of  the  heavens,  the  earth 
is  a  mere  point ;  yet,  when  we  bring  it  into  place,  its  sur- 
face cuts  off  one  half  of  the  universe  from  our  view,  just 
as  an  apple  would  cut  off  the  view  of  one  side  of  a  room 
from  an  insect  crawling  upon  it.  That  half  of  the  celes- 
tial sphere  which,  being  above  the  horizon,  remains  visi- 
ble is  called  the  visible  hemisphere;  the  half  below,  the 
view  of  which  is  cut  off  by  the  earth,  is  called  the  invisi- 
ble hemisphere.  Of  course  we  could  see  the  latter  by 
travelling  around  the  earth. 

Having  this  state  of  things  well  in  mind,  we  must 
make  another  draft  on  the  reader's  attention.  We  know 
that  the  earth  is  not  at  rest,  but  revolves  unceasingly 
around  an  axis  passing  through  its  centre.  The  natural 
result  of  this  is  an  apparent  rotation  of  the  celestial 
sphere  in  the  opposite  direction.  The  earth  rotates  from 
west  toward  east ;  hence  the  sphere  seems  to  rotate  from 
east  toward  west.  This  real  revolution  of  the  earth,  with 
the  apparent  revolution  of  the  stars  which  it  causes,  is 
called  the  diurnal  motion,  because  it  is  completed  in  a 
day. 


12  THE    CELESTIAL    MOTIONS 

Apparent  Daily  Revolution  of  the  Stars 

Our  next  problem  is  to  show  the  connection  between  the 
very  simple  conception  of  the  rotation  of  the  earth  and 
the  more  complicated  appearance  presented  by  the  ap- 
parent diurnal  motion  of  the  heavenly  bodies  which  it 
brings  about.  The  latter  varies  with  the  latitude  of  the 
observer  upon  the  earth's  surface.  Let  us  begin  with  its 
appearance  in  our  middle  northern  latitudes. 

For  this  purpose  we  may  in  imagination  build  a  hollow 
globe  representing  the  celestial  sphere.  We  may  make 
it  as  large  as  a  Ferris  wheel,  but  one  of  thirty  or  forty 
feet  in  diameter  would  answer  our  purpose.  Let  Figure 
1  be  an  inside  view  of  this  globe,  mounted  on  two  pivots, 
P  and  Q,  so  that  it  can  turn  round  on  them  diagonally. 
In  the  middle,  at  O,  we  have  a  horizontal  platform,  NS, 
on  which  we  sit.  The  constellations  are  marked  on  the 
inside  of  the  globe,  covering  the  whole  surface,  but 
those  on  the  lower  half  are  hidden  from  view  by  the 
platform.  This  platform,  as  is  evident,  represents  the 
horizon. 

The  globe  is  now  made  to  turn  on  its  pivots.  What 
will  happen?  We  shall  see  the  stars  near  the  pivot  P 
revolving  around  the  latter  as  the  globe  turns.  The 
stars  on  a  certain  circle  KN  will  graze  the  edges  of  the 
platform,  as  they  pass  below  P.  Those  yet  farther  from 
P  will  dip  below  the  platform  to  a  greater  or  less  extent, 
according  to  their  distance  from  P.  Stars  near  the  circle 
EF,  halfway  between  P  and  Q,  will  perform  half  their 
course  above,  and  half  below  the  platform.  Finally, 


REVOLUTION    OF    THE    STARS 


13 


stars  within  the  circle  ST  will  never  rise  above  the  level 
of  the  platform  at  all,  and  will  remain  invisible  to  us. 

To  our  eyes  the  celestial  sphere  is  such  a  globe  as  this, 
of  infinite  dimensions.     It  seems  to  us  to  be  continually 


VISIBLE  \     HEMISPHERE 


INVISIBLE  \HEMISPhER 


FIG.  1. —  The  Celestial  Sphere  as  it  appears  to  «s. 

revolving  round  a  certain  point  in  the  sky  as  a  pivot, 
making  one  revolution  in  nearly  a  day,  and  carrying  the 
sun,  moon,  and  stars  with  it.  The  stars  preserve  their 
relative  positions  as  if  fastened  to  the  revolving  celestial 
sphere.  That  is  to  say,  if  we  take  a  photograph  of  them 


14  THE    CELESTIAL   MOTIONS 

at  any  hour  of  the  night,  the  same  photograph  will  show 
their  appearance  at  any  other  hour,  if  we  only  hold  it  in 
the  right  position. 

The  pivot  corresponding  to  P  is  called  the  north  celes- 
tial pole.  To  dwellers  in  middle  northern  latitudes,  where 
most  of  us  live,  it  is  in  the  northern  sky,  nearly  midway 
between  the  zenith  and  the  northern  horizon.  The 
farther  south  we  live,  the  nearer  it  is  to  the  horizon,  its 
altitude  above  the  latter  being  equal  to  the  latitude  of 
the  place  where  the  observer  stands.  Quite  near  it  is  the 
pole  star,  which  we  shall  hereafter  show  how  to  locate. 
To  ordinary  observation,  the  pole  star  seems  never  to 
move  from  its  position.  In  our  time  it  is  little  more  than 
a  degree  from  the  pole,  a  quantity  with  which  we  need 
not  now  concern  ourselves. 

Opposite  the  north  celestial  pole,  and  therefore  as  far 
below  our  horizon  as  the 'north  one  is  above  it,  lies  the 
south  'celestial  pole. 

An  obvious  fact  is  that  the  diurnal  motion  as  we  see 
it  in  our  latitude  is  oblique.  When  the  sun  rises  in  the 
east  it  does  not  seem  to  go  straight  up  from  the  horizon, 
but  moves  over  toward  the  south  at  a  more  or  less  acute 
angle  with  the  horizon.  So  when  it  sets,  its  motion  rela- 
tive to  the  horizon  is  again  oblique. 

Now,  imagine  that  we  take  a  pair  of  compasses  long 
enough  to  reach  the  sky.  We  put  one  point  on  the  sky 
at  the  north  celestial  pole,  and  the  other  point  far 
enough  from  it  to  touch  the  horizon  below  the  pole. 
Keeping  the  first  point  at  the  pole  we  draw  a  complete 
circle  on  the  celestial  sphere  with  the  other  point.  This 


REVOLUTION    OF    THE    STARS  15 

circle  just  touches  the  north  horizon  at  its  lowest  point 
and,  in  our  northern  latitudes,  extends  to  near  the  zenith 
at  its  highest  point.  The  stars  within  this  circle  never 
set,  but  only  seem  to  perform  a  daily  course  around  the 
pole.  For  this  reason  this  circle  is  called  the  circle  of 
perpetual  apparition. 

The  stars  farther  south  rise  and  set,  but  perform  less 
and  less  of  their  daily  course  above  our  horizon,  till 
we  reach  the  south  point  of  it,  where  they  barely  show 
themselves. 

Stars  yet  farther  south  never  rise  at  all  in  our  lati- 
tudes., They  are  contained  within  the  circle  of  perpetual 
occupation,  which  surrounds  and  is  centred  on  the  south 
celestial  pole,  as  the  circle  of  perpetual  apparition  is 
centred  on  the  north  one. 

Figure  2  shows  the  principal  stars  of  the  northern 
heavens  within  the  circle  of  perpetual  apparition  for  the 
Northern  States.  By  holding  it  with  the  month  on  top 
we  shall  have  a  view  of  the  constellations  as  they  are  seen 
about  eight  o'clock  in  the  evening.  It  also  shows  how  to 
find  the  pole  star  in  the  centre  by  the  direction  of  the 
two  outer  stars  or  pointers  in  the  Dipper,  or  Great  Bear. 

Now  let  us  change  our  latitude  and  see  what  occurs. 
If  we  journey  toward  the  equator,  the  direction  of  ouv 
horizon  changes,  and  during  our  voyage  we  see  the  pole 
star  constantly  sinking  lower  and  lower.  As  we  ap- 
proach the  equator,  it  approaches  the  horizon,  reaching 
it  when  we  reach  the  equator.  It  is  plain  enough  that 
the  circle  of  perpetual  apparition  grows  smaller  until, 
at  the  equator,  it  ceases  to  exist,  each  pole  being  in  our 


16 


THE    CELESTIAL    MOTIONS 


horizon.  Now  the  diurnal  motion  seems  to  us  quite  dif- 
ferent from  what  it  is  here.  The  sun,  moon,  and  stars, 
when  they  rise,  commence  their  motion  directly  upwards. 
If  one  of  them  rises  exactly  in  the  east,  it  will  pass 


II X 

FIG.  2.—  The  Northern  Sky  and  the  Pole  Star. 

through  the  zenith ;  one  rising  south  of  the  east  will  pass 
south  of  the  zenith;  one  rising  north  of  the  east,  north 
of  the  zenith. 

Continuing  our  course  into  the  southern  hemisphere, 
we  find  that  the  sun,  while  still  rising  in  the  east,  gener- 
ally passes  the  meridian  to  the  north  of  the  zenith.  The 


REVOLUTION    OF    THE    STARS  17 

main  point  of  difference  between  the  two  hemispheres  is 
that,  as  the  sun  now  culminates  in  the  north,  its  ap- 
parent motion  is  not  in  the  direction  of  the  hands  of  a 
watch,  as  with  us^  but  in  the  opposite  direction.  In 
middle  southern  latitudes,  the  northern  constellations, 
so  familiar  to  us,  are  always  below  the  horizon,  but  we 
see  new  ones  in  the  south.  Some  of  these  are  noted  for 
their  beauty,  the  Southern  Cross,  for  example.  Indeed, 
it  has  often  been  thought  that  the  southern  heavens 
were  more  brilliant  and  contained  more  stars  than  the 
northern  ones.  But  this  view  is  now  found  to  be  incor- 
rect. Careful  study  and  counts  of  the  stars  show  the 
number  to  be  about  the  same  in  one  hemisphere  as  in  the 
other.  Probably  the  impression  we  have  mentioned 
arose  from  the  superior  clearness  of  the  sky  in  the 
southern  regions.  For  some  reason,  perhaps  because  of 
the  drier  climate,  the  air  is  less  filled  with  smoke  and 
haze  in  the  southern  portions  of  the  African  and  Ameri- 
can continents  than  it  is  in  our  northern  regions. 

What  we  have  said  of  the  diurnal  motion  of  the 
northern  stars  round  and  round  the  pole,  applies  to  the 
stars  in  the  southern  heavens.  But  there  is  no  southern 
pole  star,  and  therefore  nothing  to  distinguish  the  posi- 
tion of  the  southern  celestial  pole.  The  latter  has  a 
number  of  small  stars  around  it,  but  they  are  no  thicker 
than  in  any  other  region  of  the  sky.  Of  course,  the 
southern  hemisphere  has  its  circle  of  perpetual  appari- 
tion, which  is  larger  the  farther  south  we  travel.  That 
is  to  say,  the  stars  in  a  certain  circle  around  the  south 
celestial  pole  never  set,  but  simply  revolve  around  it, 


18  THE    CELESTIAL    MOTIONS 

apparently  in  an  opposite  direction  from  what  they  do 
in  the  north.  So,  also,  there  is  a  circle  of  perpetual 
occultation  containing  those  stars  around  the  north  pole 
which,  in  our  latitudes,  never  set.  After  we  go  beyond 
20°  south  latitude  we  can  no  longer  see  any  part  of  the 
constellation  Ursa  Minor.  Still  farther  south  the  Great 
Bear  will  only  occasionally  show  itself  to  a  greater  or 
less  extent  above  the  horizon. 

Could  we  continue  our  journey  to  the  south  pole  we 
should  no  longer  see  any  rising  or  setting  of  the  stars. 
The  latter  would  move  around  the  sky  in  horizontal 
circles,  the  centre  or  pole  being  at  the  zenith.  Of  course5 
the  same  thing  would  be  true  at  the  north  pole. 


HI 

RELATION  OF  TIME  AND  LONGITUDE 

WE  all  know  that  a  line  running  through  any  place 
on  the  earth  in  a  north  and  south  direction,  is  called  the 
meridian  of  that  particular  place.  More  exactly,  a  me- 
ridian of  the  earth's  surface  is  a  semicircle  passing  from 
the  north  to  the  south  pole.  Such  semicircles  pass  in 
every  direction  from  the  north  pole,  and  one  may  be 
drawn  so  as  to  pass  through  any  place.  The  meridian 
of  the  Royal  Observatory  at  Greenwich  is  now  adopted 
by  most  nations,  our  own  included,  as  the  one  from  which 
longitudes  are  njeasured,  and  by  which  in  the  United 
States  and  a  considerable  part  of  Europe  the  clocks 
are  set. 

Corresponding  to  the  terrestrial  meridian  of  a  place 
is  a  celestial  meridian  which  passes  from  the  north  celes- 
tial pole  through  the  zenith,  intersects  the  horizon  at  its 
-south  point,  and  continues  to  the  south  pole.  As  the 
earth  revolves  on  its  axis  it  carries  the  celestial  as  well  as 
the  terrestrial  meridian  with  it,  so  that  the  former,  in 
the  course  of  a  day  sweeps  over  the  whole  celestial 
sphere.  The  appearance  to  us  is  that  every  point  of  the 
celestial  sphere  crosses  the  meridian  in  the  course  of  a 
day. 

Noon  is  the  moment  at  which  the  sun  passes  the  me- 
ridian. Before  the  introduction  of  railways,  people 


20  THE    CELESTIAL    MOTIONS 

to  set  their  clocks  by  the  sun.  But  owing  to  the  obliquity 
of  the  ecliptic  and  the  eccentricity  of  the  earth's  orbit 
around  the  sun,  the  intervals  between  successive  passages 
of  the  sun  are  not  exactly  equal.  The  consequence  is 
that,  if  a  clock  keeps  exact  time,  the  sun  will  sometimes 
pass  the  meridian  before  and  sometimes  after  twelve  by 
the  clock.  When  this  was  understood,  a  distinction  was 
made  between  apparent  and  mean  time.  Apparent  time 
was  the  unequal  time  determined  by  the  sun ;  mean  time 
was  that  given  by  a  clock  keeping  perfect  time  month 
after  month.  The  difference  between  these  two  is  called 
the  equation  of  time.  Its  greatest  amounts  are  reached 
every  year  about  the  first  of  November  and  the  middle 
of  February.  At  the  former  time,  the  sun  passes  the 
meridian  sixteen  minutes  before  the  clock  shows  twelve; 
in  February,  fourteen  or  fifteen  minutes  after  twelve. 

To  define  mean  time  astronomers  imagine  a  mean  sun 
which  always  moves  along  the  celestial  equator  so  as  to 
pass  the  meridian  at  exactly  equal  intervals  of  time,  and 
which  is  sometimes  ahead  of  the  real  sun  and  sometimes 
behind  it.  This  imaginary  or  mean  sun  determines  the 
time  of  day.  The  subject  will  perhaps  be  a  little  easier 
if  we  describe  things  as  they  appear,  imagining  the  earth 
to  be  at  rest  while  the  mean  sun  revolves  around  it,  cross- 
ing the  meridian  of  every  place  in  succession.  We  thus 
imagine  noon  to  be  constantly  travelling  around  the 
world.  In  our  latitudes,  its  speed  is  not  far  from  a 
thousand  feet  per  second ;  that  is  to  say,  if  it  is  noon  at 
a  certain  place  where  we  stand,  it  will  one  second  after- 
ward be  noon  about  one  thousand  feet  farther  west,  in 


STANDARD    TIME  £1 

another  second  a  thousand  feet  yet  farther  west,  and  so 
on  through  the  twenty-four  hours,  until  noon  will  once 
more  get  back  where  we  are.  The  obvious  result  of  this 
is  that  it  is  never  the  same  time  of  day  at  the  same  mo- 
ment at  two  places  east  or  west  of  each  other.  As  we 
travel  west,  we  shall  continually  find  our  watches  to  be 
too  fast  for  the  places  which  we  reach,  while  in  travelling 
east,  they  will  be  too  slow.  This  varying  time  is  called 
local  or  astronomical  time.  The  latter  term  is  used  be- 
cause it  is  the  time  determined  by  astronomical  observa- 
tions at  any  place. 

Standard  Time 

Formerly  the  use  of  local  time  caused  great  inconven- 
ience to  travellers.  Every  railway  had  its  own  meridian 
which  it  ran  its  trains  by;  and  the  traveller  was  fre- 
quently liable  to  miss  his  train  by  not  knowing  the  rela- 
tion between  his  watch  or  a  clock  and  the  railway  time. 
So  in  1883,  our  present  system  of  standard  time  was  in- 
troduced. Under  this  system,  standard  meridians  are 
adopted  fifteen  degrees  apart,  this  being  the  space  over 
which  the  sun  passes  in  one  hour.  The  time  at  which 
'noon  passes  a  standard  meridian  is  then  used  throughout 
a  zone  extending  seven  or  eight  degrees  on  each  side. 
This  is  called  standard  time.  The  longitudes  which 
mark  the  zones  are  reckoned  from  Greenwich.  It  hap- 
pens that  Philadelphia  is  about  seventy-five  degrees  in 
longitude,  or  five  hours  in  time  from  Greenwich.  More 
exactly,  it  is  about  one  minute  of  time  more  than  this. 
Thus  the  standard  meridian  which  we  use  for  the  Middle 


22  THE    CELESTIAL    MOTIONS 

States  passes  a  little  east  of  Philadelphia.  When  mean 
noon  reaches  this  meridian,  it  is  considered  as  twelve 
o'clock  throughout  all  our  Eastern  and  Middle  States 
as  far  west  as  Ohio.  An  hour  later,  it  is  considered  twelve 
o'clock^  in  the  Mississippi  Valley.  An  hour  later,  it  is 
twelve  o'clock  for  the  regionof  the  Rocky  Mountams. 
In  yet  another  hour,  it  is  twelve  o'clock  onjthe^acific 
coast.  Thus  we  use  four  different  kinds  of  time,  Eastern 
time,  Central  time,  Mountain  time,  and  Pacific  time,  dif- 
fering from  each  other  by  entire  hours.  Using  this  time, 
the  traveller  only  has  to  set  his  watch  forward  or  back 
one  hour  at  a  time,  as  he  travels  between  the  Pacific  and 
the  Atlantic  coast,  and  he  will  always  find  it  correct  for 
the  region  in  which  he  is  at  the  time. 

It  is  by  this  difference  of  time  that  the  longitudes  of 
places  are  determined.  Imagine  that  an  observer  in 
New  York  makes  a  tap  with  a  telegraph-key  at  the  exact 
moment  when  a  certain  star  crosses  his  meridian,  and 
that  this  moment  is  recorded  at  Chicago  aS  well  as  New 
York.  When  the  star  reaches  the  meridian  of  Chicago, 
the  observer  taps  the  time  of  its  crossing  over  his  meri- 
dian in  the  same  way.  The  interval  between  the  two 
taps  shows  the  difference  of  longitude  between  the  two 
cities. 

Another  method  of  getting  the  same  result  is  for  each 
observer  to  telegraph  his  local  time  to  the  other.  The 
difference  of  the  two  times  gives  the  longitude.- 

In  this  connection,  it  must  be  remembered  that  the 
heavenly  bodies  rise  and  set  by  local,  not  standard,  time. 
Hence  the  time  of  rising  and  setting  of  the  sun,  given  in 


WHERE    THE    DAY    CHANGES  23 

the  almanacs,  will  not  answer  to  set  our  watches  by  for 
standard  time,  unless  we  are  on  one  of  the  standard 
meridians.  One  difference  between  these  two  kinds  of 
time  is  that  local  time  varies  continuously  as  we  travel 
east  or  west,  while  standard  time  varies  only  by  jumps 
of  one  hour  when  we  cross  the  boundaries  of  any  of  the 
four  zones  just  described. 

Where  the  Day  Changes 

Midnight,  like  noon,  is  continually  travelling  round 
the  earth,  crossing  all  the  meridians  in  succession.  At 
every  crossing  it  inaugurates  the  beginning  of  another 
day  on  that  meridian.  If  it  is  Monday  at  any  crossing, 
it  will  be  Tuesday  when  it  gets  back  again.  So  there 
must  be  some  meridian  where  Monday  changes  to  Tues- 
day, and  where  every  day  changes  into  the  day  follow- 
ing. This  dividing  meridian,  called  the  "date  line,"  is 
determined  only  by  custom  and  convenience.  As  colo- 
nization extended  toward  the  east  and  the  west  men 
carried  their  count  of  days  with  them.  The  result  was 
that  whenever  it  extended  so  far  that  those  going  east 
met  those  going  west  they  found  their  time  differing  by 
one  day.  What  for  the  westward  traveller  was  Monday 
was  Tuesday  for  the  eastern  one.  This  was  the  case 
when  we  acquired  Alaska.  The  Russians  having  reached 
that  region  by  travelling  east,  it  was  found  that,  when 
we  took  possession  by  going  west,  our  Saturday  was  their 
Sunday.  This  gave  rise  to  the  question  whether  the 
inhabitants,  in  celebrating  the  festivals  of  the  Greek 
Church,  should  follow  the  old  or  the  new  reckoning  of 


24  THE    CELESTIAL    MOTIONS 

days.  The  subject  was  referred  to  the  head  of  the  church 
at  St.  Petersburg,  and  finally  to  Struve,  the  director  of 
the  Pulkowa  Observatory,  the  national  astronomical  in- 
stitution of  the  empire.  Struve  made  a  report  in  favor 
of  the  American  reckoning,  and  the  change  to  it  was 
duly  carried  out. 

At  the  present  time  custom  prescribes  for  the  date  line 
the  meridian  opposite  that  of  Greenwich.  This  passes 
through  the  Pacific  Ocean,  and  in  its  course  crosses  very 
little  land — only  the  northeastern  corner  of  Asia  and, 
perhaps,  some  of  the  Fiji  Islands.  This  fortunate  cir- 
cumstance prevents  a  serious  inconvenience  which  might 
arise  if  the  date  line  passed  through  the  interior  of  a 
country.  In  this  case  the  people  of  one  city  might  have 
their  time  a  day  different  from  those  of  a  neighbouring 
city  across  the  line.  It  is  even  conceivable  that  residents 
on  two  sides  of  the  same  street  would  have  different  days 
for  Sunday.  But  being  in  the  ocean,  no  such  incon- 
venience follows.  The  date  line  is  not  necessarily  a  meri- 
dian of  the  earth,  but  may  deviate  from  one  side  to  the 
other  in  order  to  prevent  the  inconvenience  we  have 
described.  Thus  the  inhabitants  of  Chatham  Island 
have  the  same  time  as  that  of  the  neighbouring  island  of 
New  Zealand,  although  the  meridian  of  180°  from 
Greenwich  runs  between  them. 


IV 

How  THE  POSITION  OF  A  HEAVENLY  BODY  is  DEFINED 

IN  this  chapter  I  have  to  use  and  explain  some  tech- 
nical terms.  The  ideas  conveyed  by  them  are  necessary 
to  a  complete  understanding  of  the  celestial  motions,  and 
of  the  positions  of  the  stars  at  any  hour  when  we  may 
wish  to  observe  them.  To  the  reader  who  only  desires 
a  general  idea  of  celestial  phenomena,  this  chapter  will 
not  be  necessary.  I  must  invite  one  who  wants  a  knowl- 
edge more  thorough  than  this  to  make  a  close  study  of 
the  celestial  sphere  as  it  was  described  in  our  second 
chapter.  Turning  back  to  our  first  figure,  we  see  our- 
selves concerned  with  the  relation  of  two  spheres.  One 
of  these  is  the  real  globe  of  the  earth,  on  the  surface  of 
which  we  dwell,  and  which  is  continually  carrying  us 
around  by  its  daily  rotation.  The  other  is  the  apparent 
sphere  of  the  heavens,  which  surrounds  our  globe  on  all 
sides  at  an  enormous  distance,  and  which,  although  it  has 
"no  reality,  we  are  obliged  to  imagine  in  order  to  know 
where  to  look  for  the  heavenly  bodies.  Notice  that  \ve 
see  this  sphere  from  its  centre,  so  that  everything  we 
see  upon  it  appears  upon  its  inside  surface,  while  we  see 
the  surface  of  the  earth  from  the  outside. 

There  is  a  correspondence  between  points  and  circles 
on  these  two  spheres.  We  have  already  shown  how  the 
axis  of  the  earth,  which  marks  our  north  and  south  poles, 


26  THE    CELESTIAL    MOTIONS 

being  continued  in  both  directions  through  space,  marks 
the  north  and  south  poles  of  the  celestial  sphere. 

We  know  that  the  earth's  equator  passes  around  it  at 
an  equal  distance  from  the  two  poles.  In  the  same  way 
we  have  an  equator  on  the  celestial  sphere  which  passes 
around  it  at  a  distance  of  ninety  degrees  from  either 
celestial  pole.  If  it  could  be  painted  on  the  sky  we  should 
always  see  it,  by  day  or  night,  in  one  fixed  position.  We 
can  imagine  exactly  how  it  would  look.  It  intersects  the 
horizon  in  the  cast  and  west  points,  and  is  in  fact  the  line 
which  the  sun  seems  to  mark  out  in  the  sky  by  its  diurnal 
course  during  the  twelve  hours  that  it  is  above  the  hori- 
zon, in  March  or  September.  In  our  northernmost 
States,  it  passes  about  halfway  between  the  zenith  and 
the  south  horizon,  but  passes  nearer  the  zenith  the  farther 
south  we  are. 

As  we  have  circles  of  latitude  parallel  to  the  equator 
passing  around  the  earth  both  north  and  south  of  the 
equator,  so  we  have  on  the  celestial  sphere  circles  parallel 
to  the  celestial  equator,  and  therefore  having  one  or  the 
other  of  the  celestial  poles  as  a  centre.  As  the  parallels 
of  latitude  on  the  earth  grow  smaller  and  smaller  toward 
the  pole,  so  do  these  celestial  circles  grow  smaller  toward 
the  celestial  poles. 

We  know  that  longitude  on  the  earth  is  measured  by 
the  position  of  a  meridian  passing  from  the  north  to  the 
south  pole  through  the  place  whose  position  is  to  be  de- 
fined. The  angle  which  this  meridian  makes  with  that 
through  the  Greenwich  Observatory  is  the  longitude  of 
the  place. 


CIRCLES  OF  CELESTIAL  SPHERE        27 

We  have  the  same  system  in  the  heavens.  Circles  are 
imagined  to  pass  from  one  celestial  pole  to  the  other  in 
every  direction,  but  all  intersecting  the  equator  at  right 


FIG  3.—  Circles  of  tlie  Celestial  SpJiere. 

angles,  as  shown  in  Figure  3.  These  are  called  hour 
circles.  One  of  them  is  called  the  first  hour  circle,  and 
is  so  marked  in  the  figure.  It  passes  through  the  vernal 


28  THE    CELESTIAL    MOTIONS 

equinox,  a  point  to  be  defined  in  the  next  chapter.  This 
takes  a  place  in  the  sky  corresponding  to  Greenwich  on 
the  earth's  surface. 

The  position  of  a  star  on  the  celestial  sphere  is  defined 
in  the  same  way  that  the  position  of  a  city  on  the  earth 
is  defined,  by  its  latitude  and  longitude.  But  different 
terms  are  used.  In  astronomy,  the  measure  which  corre- 
sponds to  longitude  is  called  right  ascension;  that  which 
corresponds  to  latitude  is  called  declination.  We  thus 
have  the  following  definitions,  which  I  must  ask  the 
reader  to  remember  carefully. 

The  declination  of  a  star  is  its  apparent  distance  from 
the  celestial  equator  north  or  south.  In  the  figure  the 
star  is  in  declination  twenty^five  degrees  north. 

The  right  ascension  of  a  star  is  the  angle  which  the 
hour  circle  passing  through  it  makes  with  the  first  hour 
circle  which  passes  through  the  vernal  equinox.  In  the 
figure  the  star  is  in  three  hours  right  ascension. 

The  right  ascension  of  a  star  is,  in  astronomical  usage, 
generally  expresssed  as  so  many  hours,  minutes,  and 
seconds,  in  the  way  shown  on  Figure  3.  But  it  may 
equally  well  be  expressed  in  degrees  as  we  express  the 
longitude  of  places  on  the  earth.  The  right  ascension 
expressed  in  hours  may  be  changed  into  degrees  by  the 
simple  process  of  multiplication  by  15.  This  is  because 
the  earth  revolves  15°  in  an  hour.  Figure  3  also  shows 
us  that,  while  the  degrees  of  latitude  are  nearly  of 
the  same  length  all  over  the  earth,  those  of  longitude 
continually  diminish,  slowly  at  first  and  more  rapidly 
afterwards,  from  the  equator  toward  the  poles.  At  the 


POSITION    OF    A    HEAVENLY   BODY     29 

equator  the  degree  of  longitude  is  about  69|  statute  miles, 
but  at  the  latitude  of  45°  it  is  only  about  42  miles.  At 
60°  it  is  less  than  35  miles,  at  the  pole  it  comes  down  to 
nothing,  because  there  the  meridians  meet. 

We  may  see  that  the  speed  of  the  rotation  of  the 
earth  follows  the  same  law  of  diminution.  At  the  equa- 
tor, 15°  is  about  1,000  miles.  We  may  therefore  see 
that,  in  that  part  of  the  earth,  the  latter  revolves  at 
the  rate  of  1,000  miles  an  hour.  This  is  about  1,500 
feet  per  second.  But  in  latitude  45°  the  speed  is 
diminished  to  little  more  than  1,000  feet  per  second. 
At  60°,  north,  it  is  only  half  that  at  the  equator;  at  the 
poles  it  goes  down  to  nothing. 

In  applying  this  system  the  only  trouble  arises  from 
the  earth's  rotation.  As  long  as  we  do  not  travel,  we 
remain  on  the  same  circle  of  longitude  on  the  earth.  But 
by  the  rotation  of  the  earth,  the  right  ascension  of  any 
point  in  the  sky  which  seems  to  us  fixed,  is  continually 
changing.  The  only  difference  between  the  celestial 
meridian  and  an  hour  circle  is  that  the  former  travels 
round  with  the  earth,  while  the  latter  is  fixed  on  the 
celestial  sphere. 

There  is  a  strict  resemblance  in  almost  every  point 
between  the  earth  and  the  celestial  sphere.  As  the  former 
revolves  on  its  axis  from  west  to  east,  the  latter  seems  to 
revolve  from  east  to  west.  If  we  imagine  the  earth  cen- 
tred inside  the  celestial  sphere  with  a  common  axis  pass- 
ing through  them,  as  shown  in  the  figure,  we  shall  have  a 
clear  idea  of  the  relations  we  wish  to  set  forth. 

If  the  sun,  like  the  stars,  seemed  fixed  on  the  celestial 


30  THE    CELESTIAL    MOTIONS 

sphere  from  year  to  year,  the  problem  of  finding  a  star 
when  we  knew  its  right  ascension  and  declination  would 
be  easier  than  it  actually  is.  Owing  to  the  annual  revo- 
lution of  the  earth  round  the  sun  there  is  a  continual 
change  in  the  apparent  position  of  the  sphere  at  a  given 
hour  of  the  night.  We  must  next  point  out  the  effect 
of  this  revolution. 


V 
THE  ANNUAL  MOTION  OF  THE  EARTH  AND  ITS  RESULTS 

IT  is  well  known  that  the  earth  not  only  turns  on  its 
axis,  but  makes  an  annual  revolution  round  the  sun.  The 
result  of  this  motion — in  fact,  the  phenomenon  by  which 
it  is  shown — is  that  the  sun  appears  to  make  an  an- 
nual revolution  around  the  celestial  sphere  among  the 
stars.  We  have  only  to  imagine  ourselves  moving  round 
the  sun  and  therefore  seeing  the  latter  in  different  direc- 
tions, to  see  that  it  must  appear  to  us  to  move  among 
the  stars,  which  are  farther  than  it  is.  It  is  true  that 
the  motion  is  not  at  once  evident  because  the  stars  are 
invisible  in  the  daytime.  But  the  fact  of  the  motion 
will  be  made  very  clear  if,  day  after  day,  we  watch  some 
particular  fixed  star  in  the  west.  We  shall  find  that  it 
sets  earlier  and  earlier  every  day;  in  other  words,  it  is 
getting  continually  nearer  and  nearer  the  sun.  More 
exactly,  since  the  real  direction  of  the  star  is  unchanged, 
the  sun  appears  to  be  approaching  the  star. 

If  we  could  see  the  stars  in  the  daytime,  all  round  the 
sun,  the  case  would  be  yet  clearer.  We  should  see  that 
if  the  sun  and  a  star  rose  together  in  the  morning  the 
sun  would,  during  the  day,  gradually  work  past  the  star 
in  an  easterly  direction.  Between  the  rising  and  setting 
it  would  move  nearly  its  own  diameter  relative  to  the  star. 
Next  morning  we  should  see  that  it  had  gotten  quite 


32  THE    CELESTIAL    MOTIONS 

away  from  the  star,  being  nearly  two  diameters  distant 
from  it.  The  figure  shows  how  this  would  go  on  at  the 
time  of  the  spring  equinox,  after  March  twentieth.  This 
motion  would  continue  month  after  month.  At  the  end 


2     v  '  ' 


•;•    ;-;.     •  ;  •.    ///-  •      ,  ;     •;     v;:-'  v.    j  .       v  \'v,  M  A  D  r'u 

FIG.  4.  —  The  Sun  Crossing  the  Equator  about  March  Twentieth. 

of  the  year  the  sun  would  have  made  a  complete  circuit 
of  the  heavens  relative  to  the  star,  and  we  should  see  the 
two  once  more  together. 


The  Sun's  Apparent  Path 

How  the  above  effect  is  produced  will  be  seen  by  Fig- 
ure 5,  which  represents  the  earth's  orbit  round  the  sun, 
with  the  stars  in  the  vast  distance.  When  the  earth  is  at 
A,  we  see  the  sun  in  the  line  AM,  as  if  it  were  among  the 
stars  at  M.  As  we  are  carried  on  the  earth  from  A  to  B, 
the  sun  seems  to  move  from  M  to  N,  and  so  on  through 
the  year.  This  apparent  motion  of  the  sun  in  one  year 
around  the  celestial  sphere,  was  noticed  by  the  ancients, 
who  seem  to  have  taken  much  trouble  to  map  it  out.  They 


THE    SUN'S    APPARENT    PATH  33 

imagined  a  line  passing  around  the  celestial  sphere  which 
the  sun  always  followed  in  its  annual  course,  and  which 
was  called  the  ecliptic.  They  noticed  that  the  planets 
followed  nearly  but  not  exactly  the  same  general  course 
as  the  sun  among  the  stars.  A  belt  extending  around  on 


*  *  o* 

CAPB^0^ 

FIG.  5.—  Tlie  Orbit  of  the  Earth  and  tJie  Zodiac. 

each  side  of  the  ecliptic,  and  broad  enough  to  contain 
all  the  known  planets,  as  well  as  the  sun,  was  called  the 
zodiac.  It  was  divided  into  twelve  signs,  each  marked 
by  a  constellation.  The  sun  went  through  each  sign  in 
the  course  of  a  month  and  through  all  twelve  signs  in  a 
year.  Thus  arose  the  familiar  signs  of  the  zodiac,  which 


34  THE    CELESTIAL    MOTIONS 

bore  the  same  names  as  the  constellations  among  which 
they  were  situated.  This  is  not  the  case  at  present, 
owing  to  the  slow  motion  of  precession  soon  to  be 
described. 

It  will  be  seen  that  the  two  great  circles  we  have  de- 
scribed spanning  the  entire  celestial  sphere  are  fixed  in 
entirely  different  ways.  The  equator  is  determined  by 
the  direction  in  which  the  axis  of  the  earth  points,  and 
spans  the  sphere  midway  between  the  two  celestial  poles. 
The  ecliptic  is  determined  by  the  earth's  motion  around 
the  sun. 

These  two  circles  do  not  coincide,  but  intersect  each 
other  at  two  opposite  points,  at  an  angle  of  twenty-three 
and  a  half  degrees,  or  nearly  one  quarter  of  a  right 
angle.  This  angle  is  called  the  obliquity  of  the  ecliptic. 
To  understand  exactly  how  it  arises  we  must  mention 
a  fact  about  the  celestial  poles ;  from  what  we  have  said 
of  them  it  will  be  seen  that  they  are  not  determined  by 
anything  in  the  heavens,  but  by  the  direction  of  the 
earth's  axis  only;  they  are  nothing  but  the  two  op- 
posite points  in  the  heavens  which  lie  exactly  in  the 
line  of  the  earth's  axis.  The  celestial  equator,  being 
the  great  circle  halfway  between  the  poles,  is  also 
fixed  by  the  direction  of  the  earth's  axis  and  by  nothing 
else. 

Let  us  now  suppose  that  the  earth's  orbit  around  the 
sun  is  horizontal.  We  may  in  imagination  represent 
it  by  the  circumference  of  a  round  level  platform  with 
the  sun  in  its  centre.  We  suppose  the  earth  to  move 
around  the  circumference  of  the  platform  with  its  cen- 


THE    SUN'S    APPARENT    PATH  35 

trc  on  the  level  of  the  platform ;  then,  if  tne  earths  axis 
were  vertical,  its  equator  would  be  horizontal  and  on  a 
level  with  the  platform  and  therefore  would  always  be 
directed  toward  the  sun  in  its  centre,  as  the  earth  made  its 
annual  course  around  the  platform.  Then,  on  the  celes- 
tial sphere,  the  ecliptic  determined  by  the  course  of  the 
sun  would  be  the  same  circle  as  the  equator.  The 
obliquity  of  the  ecliptic  arises  from  the  fact  that  the 
earth's  orbit  is  not  vertical-,  as  just  supposed,  but  is  in- 

/A^. 


FIG.  6. — How  the  Obliquity  of  the  Ecliptic  Produces  the  Changes  of  Seasons. 

clined  twenty-three  and  a  half  degrees.  The  ecliptic 
has  the  same  inclination  to  the  plane  of  the  platform; 
thus  the  obliquity  is  the  result  of  the  inclination  of  the 
earth's  axis.  An  important  fact  connected  with  the  sub- 
ject is  that,  as  the  earth  makes  its  revolutions  around  the 
sun,  the  direction  of  its  axis  remains  unchanged  in  space ; 
hence  its  north  pole  is  tipped  away  from  the  sun  or 
toward  it,  according  to  its  position  in  the  orbit.  This 
is  shown  in  Figure  6,  which  represents  the  platform  we 
have  supposed,  with  the  axis  tipped  toward  the  right 
hand.  The  north  pole  will  always  be  tipped  in  this 


36 


THE    CELESTIAL    MOTIONS 


direction,  whether  the  earth  is  east,  west,  north,  or  south 
from  the  sun. 

To  see  the  effect  of  the  inclination  upon  the  ecliptic 
suppose  that,  at  noon  on  some  twenty-first  day  of  March, 
the  earth  should  suddenly  stop  turning  on  its  axis,  but 
continue  its  course  around  the  sun.  What  we  should  then 
see  during  the  next  three  months  is  represented  in  Figure 
7,  in  which  we  are  supposed  to  be  looking  at  the  southern 
sky.  We  see  the  sun  on  the  meridian,  where  it  will  at 
first  seem  to  remain  immovable.  The  figure  shows  the 


FIG.  7. — Apparent  Motion  of  the  Sun  along  the  Ecliptic  in  Spring  and 
Summer. 

celestial  equator  passing  through  the  east  and  west 
points  of  the  horizon  as  already  described  and  also  the 
ecliptic,  intersecting  it  at  the  equinox.  Watching  the 
result  for  a  time  equal  to  three  of  our  months  we  should 
see  the  sun  slowly  make  its  way  along  the  ecliptic  to 
the  point  marked  "summer  solstice,"  its  farthest  north- 
ern point,  which  it  would  reach  about  June  twentieth. 

Figure  8  enables  us  to  follow   its   course  for  three 
months  longer.     After  passing  the  summer  solstice,  its 


THE    SUN'S    APPARENT    PATH          37 

course  gradually  carries  it  once  more  to  the  equator, 
which  it  again  crosses  about  September  twentieth.  Its 
course  during  the  rest  of  the  year  is  the  counterpart  of 
that  during  the  first  six  months.  It  is  farthest  south  of 
the  equator  on  December  twentieth,  and  again  crosses  it 
on  March  twentieth. 

We  see  that  there  are  four  cardinal  points  in  this  ap- 
parent annual  course  of  the  sun.  (1)  Where  we  have 
commenced  our  watch  is  the  vernal  equinox.  (£)  The 
point  where  the  sun,  having  reached  its  northern  limit, 
begins  to  again  approach  the  equator.  This  is  called  the 
summer  solstice.  (  3  )  Opposite  the  vernal  equinox  is  the 


FIG.  8. — Apparent  Motion  of  the  Sun  from  March  till  September. 

autumnal  equinox,  which  the  sun  passes  about  September 
twentieth.  (4?)  Opposite  the  summer  solstice  is  the  point 
where  the  sun  is  farthest  south.  This  is  called  the  winter 
solstice. 

The  hour  circles  which  pass  from  one  celestial  pole  to 
the  other  through  these  points  at  right  angles  to  the 
equator  are  called  colures.  That  which  passes  through 


38  THE    CELESTIAL    MOTIONS 

the  vernal  equinox  is  the  first  meridian,  from  which  right 
ascensions  are  counted  as  already  described.  The  two 
at  right  angles  to  it  are  called  the  solstitial  colures. 
'  Let  us  now  show  the  relation  of  the  constellations  to 
the  seasons  and  the  time  of  day.  Suppose  that  to-day 
the  sun  and  a  star  passed  the  meridian  at  the  same  mo- 
ment; to-morrow  the  sun  will  be  nearly  a  degree  to  the 
east  of  the  star,  which  shows  that  the  star  will  pass  the 
meridian  nearly  four  minutes  sooner  than  the  sun  will. 
This  will  continue  day  after  day  throughout  the  entire 
year  when  the  two  will  again  pass  the  meridian  at  about 
the  same  moment.  Thus  the  star  will  have  passed  once 
oftener  than  the  sun.  That  is  to  say:  In  the  course 
of  a  year  while  the  sun  has  passed  the  meridian  three 
hundred  and  sixty-five  times,  a  star  has  passed  it  three 
hundred  and  sixty-six  times.  Of  course  if  we  take  a 
star  in  the  south  it  will  have  risen  and  set  the  same 
number  of  times. 

Astronomers  keep  the  reckoning  of  this  different  ris- 
ing and  setting  of  the  stars  by  using  a  sidereal  day,  or 
star  day,  equal  to  the  interval  between  two  passages  of 
a  star,  or  of  the  vernal  equinox,  across  the  meridian.  They 
divide  this  day  into  twenty-four  sidereal  hours,  and  these 
into  minutes  and  seconds  according  to  the  usual  plan. 
They  also  use  sidereal  clocks  which  gain  about  three 
minutes  and  fifty-six  seconds  per  day  on  the  ordinary 
clocks,  and  thus  show  sidereal  time.  Sidereal  noon  is  the 
moment  at  which  the  vernal  equinox  crosses  the  meridian 
of  the  place.  The  clock  is  then  set  at  0  hours,  0  minutes, 
nd  0  seconds.  Thus  set  and  regulated,  the  sidereal 


THE    SEASONS  39 

clock  keeps  time  with  the  apparent  rotation  of  the  celes- 
tial sphere,  so  that  the  astronomer  has  only  to  look  at  his 
clock  to  see,  by  day  or  by  night,  what  stars  are  on  the 
meridian  and  what  the  positions  of  the  constellations  are. 

The  Seasons 

If  the  earth's  axis  were  perpendicular  to  the  plane  of 
the  ecliptic,  the  latter  would  coincide  with  the  equator, 
and  we  should  have  no  difference  of  seasons  the  year 
round.  The  sun  would  always  rise  in  the  exact  east  and 
set  in  the  exact  west.  There  would  be  only  a  very  slight 
change  in  the  temperature  arising  from  the  fact  that 
the  earth  is  a  little  nearer  the  sun  in  January  than  in 
July.  Owing  to  the  obliquity  of  the  ecliptic  it  folio  we 
that,  while  the  sun  is  north  of  the  equator,  which  is  the 
case  from  March  to  September,  the  sun  shines  upon  the 
northern  hemisphere  during  a  greater  time  of  each  day 
and  at  a  greater  angle,  than  on  the  southern  hemisphere. 
In  the  southern  hemisphere  the  opposite  is  the  case.  The 
sun  shines  longer  from  September  till  March  than  it  does 
on  the  northern  hemisphere.  Thus  we  have  winter  in 
the  northern  hemisphere  when  it  is  summer  in  the 
southern,  and  lice  versa. 

Relations  between  Real  and  Apparent  Motions 

Before  going  farther  let  us  recapitulate  the  phenom- 
ena we  have  described  from  the  two  points  of  view:  one 
that  of  the  real  motions  of  the  earth ;  the  other  that  of 
the  apparent  motions  of  the  heavens,  to  which  the  real 
motions  give  rise. 


40  THE    CELESTIAL    MOTIONS 

The  real  diurnal  motion  is  the  turning  of  the  earth  on 
its  axis. 

The  apparent  diurnal  motion  is  that  which  the  stars 
appear  to  have  in  consequence  of  the  earth's  rotation. 

The  real  annual  motion  is  that  of  the  earth  round  the 
sun. 

The  apparent  annual  motion  is  that  of  the  sun  around 
the  celestial  sphere  among  the  stars. 

By  the  real  diurnal  motion  the  plane  of  our  horizon  is 
carried  past  the  sun  or  a  star. 

We  then  say  that  the  sun  or  star  rises  or  sets,  as  the 
case  may  be. 

About  March  twenty-first  of  every  year  the  plane  of 
the  earth's  equator  passes  from  the  north  to  the  south 
of  the  sun,  and  about  September  twenty-first  it  repasses 
toward  the  north. 

We  then  say  that  the  sun  crosses  to  the  north  of  the 
equator  in  March,  and  to  the  south  in  September. 

In  June  of  every  year  the  plane  of  the  earth's  equator 
is  at  the  greatest  distance  south  of  the  sun,  and  in  De- 
cember at  the  greatest  distance  north. 

We  say  in  the  first  case  that  the  sun  is  at  the  north- 
ern solstice,  and  in  the  second  that  it  is  at  the  southern 
solstice. 

The  earth's  axis  is  tipped  twenty-three  and  a  half 
degrees  from  the  perpendicular  to  the  earth's  orbit. 

The  apparent  result  is  that  the  ecliptic  is  inclined 
twenty-three  and  a  half  degrees  to  the  celestial  equator. 

During  June  and  the  other  summer  months  the  north- 
ern hemisphere  of  the  earth  is  tipped  toward  the  sun, 


PRECESSION    OF    THE    EQUINOXES      41 

Places  in  north  latitude,  as  they  are  carried  round  by 
the  turning  of  the  earth,  are  then  in  sunlight  during 
more  than  half  their  course ;  those  in  south  latitude  less. 

The  result  as  it  appears  to  us  is  that  the  sun  is  more 
than  half  the  time  above  the  horizon,  and  that  we  have 
the  hot  weather  of  summer,  while  in  the  southern  hemi- 
sphere the  days  are  short,  and  the  season  is  winter. 

During  our  winter  months  the  case  is  reversed.  The 
southern  hemisphere  is  then  tipped  toward  the  sun,  and 
the  northern  hemisphere  away,  from  it.  Consequently, 
summer  and  long  days  are  the  order  in  the  southern,  and 
the  reverse  in  the  northern  hemisphere. 

The  Year  and  the  Precession  of  the  Equinoxes 

We  most  naturally  define  the  year  as  the  interval  of 
time  in  which  the  earth  revolves  around  the  sun.  From 
what  we  have  said,  there  are  two  ways  of  ascertaining  its 
length.  One  is  to  find  the  interval  between  two  passages 
of  the  sun  past  the  same  star.  The  other  is  to  find  the 
interval  between  two  passages  of  the  sun  past  the  same 
equinox,  that  is,  across  the  equator.  If  the  latter  were 
fixed  among  the  stars  the  two  intervals  would  be  equal. 
But  it  was  found  by  the  ancient  astronomers,  from  obser- 
vations extending  through  several  centuries,  that  these 
two  methods  did  not  give  the  same  length  of  year.  It 
took  the  sun  about  eleven  minutes  longer  to  make  the 
circuit  of  the  stars  than  to  make  the  circuit  of  the 
equinoxes.  This  shows  that  the  equinoxes  steadily 
shift  their  position  among  the  stars  from  year  to  year. 
This  shift  is  called  the  precession  of  the  equinoxes.  It 


42  THE    CELESTIAL    MOTIONS 

does  not  arise  from  anything  going  on  in  the  heavens,  but 
only  from  a  slow  change  in  the  direction  of  the  earth's 
axis  from  year  to  year  as  it  moves  around  the  sun. 

If  we  should  suppose  the  platform  in  Figure  6  to  last 
for  six  or  seven  thousand  years,  and  the  earth  to  make  its 
six  or  seven  thousand  revolutions  around  it,  we  should 
find  that,  at  the  end  of  this  time,  the  north  end  of  the 
axis  of  the  earth,  instead  of  being  tipped  toward  our 
right  hand,  as  shown  in  the  figure,  would  be  tipped 
directly  toward  us.  At  the  end  of  another  six  or  seven 
thousand  years  it  would  be  tipped  toward  our  left ;  at  the 
end  of  a  third  such  period  it  would  be  tipped  away  from 
us,  and  at  the  end  of  a  fourth,  or  about  twenty-six 
thousand  years  in  all,  it  would  have  gotten  back  to  its 
original  direction.  Since  the  celestial  poles  are  deter- 
mined by  the  direction  of  the  earth's  axis,  this  change  in 
the  direction  of  the  axis  makes  them  slowly  go  around  a 
circle  in  the  heavens,  having  a  radius  of  about  twenty- 
three  and  a  half  degrees.  At  the  present  time  the  pole 
star  is  a  little  more  than  a  degree  from  the  pole.  But 
the  pole  is  gradually  approaching  it  and  will  pass  by  it 
in  about  two  hundred  years.  In  twelve  thousand  years 
from  now  the  pole  will  be  in  the  constellation  Lyra,  about 
five  degrees  from  the  bright  star  Vega  of  that  constella- 
tion. In  the  time  of  the  ancient  Greeks  their  navigators 
did  not  recognize  any  pole  star  at  all,  because  what  is 
now  such  was  then  ten  or  twelve  degrees  from  the  pole, 
the  latter  having  been  between  it  and  the  constellation  of 
the  Great  Bear.  It  was  the  latter  which  they  steered  by, 
and  which  they  called  the  Cynosure. 


LENGTH    OF    THE    YEAR  43 

It  follows  from  all  this  that,  since  the  celestial  equator 
is  the  circle  midway  between  the  two  poles,  there  must 
be  a  corresponding  shift  in  its  position  among  the  stars. 
The  effect  of  this  shift  during  the  past  two  thousand 
years  is  shown  in  Figure  9.  Since  the  equinoxes  are  the 
points  of  crossing  of  the  ecliptic  and  the  equator,  they 
also  change  in  consequence  of  this  motion.  It  is  thus 
that  the  precession  of  the  equinoxes  arises. 


2OOO  YEARS  AGO 

*         * 

CELESTIAL    EQUATOR    20OO    YEARS    AGO 
CONSTELLATION     PISCES 

^ 

CELESTIAL    EQUATOR 


FIG.  9. — Precession  of  the  Equinoxes. 

The  two  kinds  of  year  we  have  described  are  called 
equinoctial  and  sidereal.  The  equinoctial  year,  also 
called  the  solar  year,  is  the  interval  between  two  returns 
of  the  sun  to  the  equinox.  Its  length  is — 

365  days  5  hours  48  minutes  46  seconds. 

Since  the  seasons  depend  upon  the  sun's  being  north 
or  south  of  the  equator,  the  solar  or  equinoctial  year  is 
that  used  in  the  reckoning  of  time.  The  ancient  astrono- 
mers found  that  its  length  was  about  three  hundred 
and  sixty-five  and  one  quarter  days.  As  far  back  as  the 
time  of  Ptolemy  the  length  of  the  year  was  known  even 
more  exactly  than  this,  and  found  to  be  a  few  minutes 
less  than  three  hundred  and  sixty-five  and  one  quarter 
days.  The  Gregorian  Calendar,  which  nearly  all  civi- 


44  THE    CELESTIAL    MOTIONS 

lised  nations  now  use,  is  based  upon  a  close  approxima- 
tion to  this  length  of  the  year. 

The  sidereal  year  is  the  interval  between  two  passages 
of  the  sun  past  the  same  star.  Its  length  is  three  hun- 
dred and  sixty-five  days  six  hours  and  nine  minutes. 

According  to  the  Julian  calendar9  which  was  in  use 
in  Christendom  until  1582,  the  year  was  considered  to 
be  exactly  365J  days.  This,  it  will  be  seen,  was  11 
minutes  14  seconds  more  than  the  true  length  of  the 
solar  year.  Consequently,  the  seasons  were  slowly 
changing  in  the  course  of  centuries.  In  order  to  obviate 
this,  and  have  a  year  whose  average  length  was  as  nearly 
as  possible  correct,  a  decree  was  passed  by  Pope  Gregory 
XIII  by  which,  in  three  centuries  out  of  four,  a  day 
was  dropped  from  the  Julian  calendar.  According  to 
the  latter,  the  closing  year  of  every  century  would  be 
a  leap  year.  In  the  Gregorian  calendar  1600  was  still 
to  remain  a  leap  year,  but  15009  1700,  1800,  and  1900 
were  all  common  years. 

The  Gregorian  calendar  was  adopted  immediately  by 
all  Catholic  countries,  and  from  time  to  time  by  Protes- 
tant countries  also,  so  that  for  the  past  150  years  it  has 
been  universal  in  both.  But  Russia  has  held  on  to  the 
Julian  calendar  until  this  day.  Consequently  in  that 
country  the  reckoning  of  time  is  now  13  days  behind 
that  in  the  other  Christian  countries.  The  Russian  New 
Year  of  1900  occurred  on  what  we  call  January  13.  In 
February  of  that  year  we  only  counted  28  days,  but 
Russia  counted  29.  Hence,  in  1901,  the  Russian  New 
Year  was  carried  still  farther  forward  to  our  January  14. 


PART   II 
ASTRONOMICAL   INSTRUMENTS 


I 

THE  REFRACTING  TELESCOPE 

THERE  is  no  branch  of  science  more  interesting  to  the 
public  than  that  with  which  the  telescope  is  concerned.  I 
assume  that  the  reader  wishes  to  have  an  intelligent  idea 
as  to  what  a  telescope  is  and  what  can  be  seen  with  it. 
In  its  most  complete  form,  as  used  by  the  astronomer  in 
his  observatory,  the  instrument  is  quite  complex.  But 
there  are  a  few  main  points  about  it  which  can  be  mas- 
tered in  a  general  way  by  a  little  close  attention.  After 
mastering  these  points,  the  visitor  to  an  observatory  will 
examine  the  instrument  with  much  more  satisfaction  than 
he  can  when  he  knows  nothing  about  it. 

The  one  great  function  of  a  telescope,  as  we  all  know, 
is  to  make  distant  objects  look  nearer  to  us;  to  see  an 
object  miles  away  as  if  it  were,  perhaps,  only  as  many 
yards.  The  optical  appliances  by  which  this  is  effected 
are  extremely  simple.  They  are  made  with  large  well- 
polished  lenses,  of  the  same  kind  as  those  used  in  a  pair 
of  spectacles,  differing  from  the  latter  only  in  their  size 
and  general  perfection.  A  telescope  requires  an  ap- 
pliance for  collecting  the  light  coming  from  the  object 
so  as  to  form  an  image  of  the  latter.  There  are  two 
ways  in  which  the  light  may  be  collected,  one  by  passing 
the  light  through  a  set  of  lenses,  and  one  by  reflecting  it 
from  a  concave  mirror.  Thus  we  have  two  different  kinds 


48          ASTRONOMICAL    INSTRUMENTS 

of  telescope,  one  called  refracting,  the  other  reflecting. 
We  begin  with  the  former  because  it  is  the  more  usual. 

The  Lenses  cf  a  Telescope 

The  lenses  of  a  refracting  telescope  comprise  two  com- 
binations or  systems;  the  one  an  object-glass — or  "ob- 
jective," as  it  is  sometimes  called  for  shortness — which 
forms  the  image  of  a  distant  object  in  the  focus  of  the 
instrument;  and  the  other  an  eyepiece,  with  which  this 
image  is  viewed. 

The  objective  is  the  really  difficult  and  delicate  part 
of  the  instrument.  Its  construction  involves  more  refined 
skill  than  that  of  all  the  other  parts  together.  How  great 
is  the  natural  aptitude  required  may  be  judged  from  the 
fact  that  a  generation  ago  there  was  but  one  man  in  the 
world  in  whose  ability  to  make  a  perfect  object-glass  of 
the  largest  size  astronomers  everywhere  would  have  felt 
confidence.  This  man  was  Alvan  Clark,  of  whom  we 
shall  soon  speak. 

The  object-glass,  as  commonly  made,  consists  of  two 
large  lenses.  The  power  of  the  telescope  depends  alto- 
gether on  the  diameter  of  these  lenses,  which  is  called 
the  aperture  of  the  telescope.  The  aperture  may  vary 
from  three  or  four  inches,  in  the  little  telescope  which 
one  has  in  his  house,  to  more  than  three  feet  in  the  great 
telescope  of  the  Yerkes  Observatory.  One  reason  why 
the  power  of  the  telescope  depends  on  the  diameter 
of  the  object-glass  is  that,  in  order  to  see  an  object  mag- 
nified a  certain  number  of  times,  in  its  natural  bright- 
ness, we  need  a  quantity  of  light  expressed  by  the  square 


THE    LENSES    OF    A    TELESCOPE        49 

of  the  magnifying  power.  For  example,  if  we  have  a 
magnifying  power  of  one  hundred,  we  should  need  ten 
thousand  times  the  light.  I  do  not  mean  that  this  quan- 
tity of  light  is  always  necessary ;  it  is  not  so,  because  we 
can  commonly  see  an  object  with  less  than  its  natural 
illumination.  Still,  we  need  a  certain  amount  of  light, 
or  it  will  be  too  dim. 

In  order  that  distinct  vision  of  a  distant  object  may 
be  secured  in  the  telescope,  the  one  great  essential  is  that 
the  object-glass  should  bring  all  the  rays  coming  from 
any  one  point  of  the  object  observed  to  the  same  focus. 
If  this  is  not  brought  about;  if  different  rays  come  to 
slightly  different  foci,  then  the  object  will  look  blurred, 
as  if  it  were  seen  through  a  pair  of  spectacles  which  did 
not  suit  our  eyes.  Now,  a  single  lens,  no  matter  of  what 
sort  of  glass  we  make  it,  will  not  bring  rays  to  the  same 
focus.  The  reader  is  doubtless  aware  that  ordinary  light, 
whether  coming  from  the  sun  or  a  star,  is  of  a  countless 
multitude  of  different  colours,  which  can  be  separated  by 
passing  the  light  through  a  triangular  prism.  These 
colours  range  from  red  at  one  end  of  the  scale,  through 
yellow,  green,  and  blue,  to  violet  at  the  other.  A  single 
lens  brings  these  different  rays  to  different  foci ;  the  red 
farthest  from  the  object-glass;  the  violet  nearest  to  it. 
This  separation  of  the  rays  is  called  dispersion. 

The  astronomers  of  two  centuries  ago  found  it  impos- 
sible to  avoid  the  dispersion  of  a  lens.  About  1750, 
Dollond,  of  London,  found  that  it  was  possible  to  cor- 
rect this  defect  by  using  two  different  kinds  of  glass, 
the  one  crown  glass  and  the  other  flint  glass.  The  prin- 


50          ASTRONOMICAL    INSTRUMENTS 

ciple  by  which  this  is  done  is  very  simple.  Crown  glass 
has  nearly  the  same  refracting  power  as  flint,  but  it  has 
nearly  twice  the  dispersive  power.  So  Dollond  made 
an  objective  of  two  lenses,  a  section  of  which  is  shown  in 
the  figure.  First  there  was  a  convex  lens  of  crown  glass, 
which  is  of  the  usual  construction.  Combined  with  this 
is  a  concave  lens  of  flint  glass.  These  two  lenses,  being 
of  opposite  curvatures,  act  on  the  light  in  opposite  direc- 
tions. The  crown  glass  tends  to  bring  the  light  to  a 
focus,  while  the  flint,  being  concave,  would  make  the  rays 
diverse.  If  it  were  used  alone,  we 
should  find  that  the  rays  passing 
through  it,  instead  of  coming  to  a 
FIG.  10.— Section  of  the  focus,  diverge  farther  and  farther 
Object-glass  of  a  Tele-  f  rom  a  f  ocus  in  different  direc- 
scope. 

tions.     Now,  the  flint  glass  is  made 

with  but  little  more  than  half  the  power  of  the  crown. 
This  half  power  is  sufficient  to  neutralize  the  dispersion 
of  the  crown ;  but  it  does  not  neutralize  much  more  than 
half  the  refraction.  The  combined  result  is  that  all  the 
rays  passing  through  the  combination  are  brought  nearly 
to  one  focus,  which  is  about  twice  as  far  away  as  the 
focus  of  the  crown  alone. 

I  say  brought  nearly  to  one  focus.  It  happens,  un- 
fortunately, that  the  combined  action  of  the  two  glasses 
is  such  that  it  is  impossible  to  bring  all  the  rays  of  the 
various  colours  absolutely  to  the  same  focus.  The  diver- 
gence, in  the  case  of  the  brighter  rays,  can  be  made  very 
small  indeed,  but  it  cannot  be  cured  entirely.  The  larger 
the  telescope,  the  more  serious  the  defect.  If  you  look 


THE  IMAGE  OF  A  DISTANT  OBJECT     51 

at  a  bright  star  through  any  large  refracting  telescope, 
you  will  see  it  surrounded  by  a  blue  or  purple  radiance. 
This  is  produced  by  the  blue  or  violet  light  which  the 
two  lenses  will  not  bring  to  one  focus. 

The  Image  of  a  Distant  Object 

By  the  action  of  the  objective,  in  thus  bringing  rays 
to  a  focus,  the  image  of  a  distant  object  is  formed  in 
the  focal  plane.  This  is  a  plane  passing  through  the 
focus  at  right  angles  to  the  axis  or  line  of  sight  of  the 
telescope. 

What  is  meant  by  the  image  formed  by  a  telescope  can 
be  seen  by  looking  into  the  ground  glass  of  a  camera  with 
the  photographer,  as  he  sets  his  instrument  for  a  picture. 
You  there  see  a  face  or  a  distant  landscape  pictured  on 
the  ground  glass.  To  all  intents  and  purposes  the 
camera  is  a  small  telescope,  and  the  ground  glass,  or  the 
point  where  the  sensitive  plate  is  to  be  fixed  to  take  a 
picture,  is  the  focal  plane.  We  may  state  the  matter 
in  the  reverse  direction  by  saying  that  the  telescope  is  a 
large  camera  of  long  focus,  with  which  we  can  take 
photographs  of  the  heavens  as  the  photographer  takes 
ordinary  pictures  with  the  camera. 

Sometimes  we  can  better  comprehend  what  an  object 
is  by  understanding  what  it  is  not.  In  the  celebrated 
moon  hoax  of  half  a  century  ago  or  more,  there  was  a 
statement  which  illustrates  what  an  image  is  not.  The 
writer  said  that  Sir  John  Herschel  and  his  friend  finding 
that,  when  they  used  enormous  magnifying  power,  there 
was  not  light  enough  for  the  image  to  be  visible,  the 


52         ASTRONOMICAL    INSTRUMENTS 

friend  suggested  that  the  image  should  be  illuminated 
by  artificial  light.  This  was  done  with  such  brilliant 
success  that  animals  in  the  moon  were  made  visible 
through  the  telescope.  If  many  people,  even  those  of 
the  greatest  intelligence,  had  not  been  deceived  by  this, 
I  should  hardly  deem  it  necessary  to  say  that  the  image 
of  an  object  formed  by  a  telescope  is  such  that,  in  the 
very  nature  of  things,  extraneous  light  cannot  aid  in  its 
formation.  Its  effectiveness  does  not  proceed  from  its 
being  a  real  image,  but  only  from  the  fact  that  all  the 
rays  from  any  one  point  of  a  distant  object  meet  in  a 
corresponding  point  of  the  image,  and  there  diverge 
again,  just  as  if  a  picture  of  the  object  were  placed  in 
the  focal  plane.  The  fact  is  that  the  term  picture  is 
perhaps  a  little  better  one  than  image  to  apply  to  this 
representation  of  the  object,  only  the  picture  is  formed 
by  light  and  nothing  else. 

If  an  image  or  picture  of  the  object  is  thus  formed 
so  as  to  stand  out  before  our  eyes,  one  may  ask  why  an 
eyepiece  is  necessary  to  view  it ;  why  the  observer  cannot 
stand  behind  the  picture,  look  toward  the  objective  and 
see  the  picture  hanging  in  the  air,  as  it  were.  He  can 
really  do  so  if  he  holds  a  ground  glass  in  the  focal  plane, 
as  the  photographer  does  with  the  camera.  He  can  thus 
see  the  image  formed  on  the  glass.  If  he  looks  into  the 
object-glass  he  can  see  it  without  any  eyepiece.  But 
only  a  very  small  portion  of  it  will  be  visible  at  any  one 
point,  and  the  advantage  over  looking  directly  at  the 
object  will  be  slight.  To  see  it  to  advantage  an  eyepiece 
must  be  used.  This  is  nothing  more  than  a  little  eye 


POWER  AND  DEFECTS  OF  TELESCOPE   53 

glass,  essentially  of  the  same  kind  that  the  watchmaker 
uses  to  examine  the  works  of  a  watch.  The  smaller  the 
eyepiece,  the  more  closely  the  examination  can  be  made, 
and  the  greater  the  magnifying  power. 

Power  and  Defects  of  a  Telescope 

The  question  is  often  asked,  how  great  is  the  magnify- 
ing power  of  some  celebrated  telescope.  The  answer  is 
that  the  magnifying  power  depends  not  only  on  the 
object-glass  but  on  the  eyepiece.  The  smaller  the  latter 
the  greater  the  magnifying  power.  Astronomical  tele- 
scopes are  supplied  with  quite  a  large  collection  of  eye- 
pieces, varying  from  the  lowest  to  the  highest  power, 
according  to  the  needs  of  the  observer. 

So  far  as  the  geometric  principle  goes,  we  can  get  any 
magnifying  power  we  please  on  any  telescope,  however 
small.  By  viewing  the  image  with  an  ordinary  micro- 
scope, such  as  is  used  by  physicians,  we  might  give  a 
little  four-inch  telescope  the  magnification  of  Herschel's 
great  reflectors.  But  there  are  many  practical  difficul- 
ties in  carrying  the  magnification  of  any  instrument 
above  a  certain  point.  First  there  is  the  want  of  light 
in  seeing  the  surface  of  an  object.  If  we  looked  at 
Saturn  with  a  three-inch  telescope,  using  a  magnifying 
power  of  several  hundred  times,  the  planet  would  seem 
dim  and  indistinct.  But  this  is  not  the  only  difficulty 
in  using  a  high  magnifying  power  with  a  small  telescope. 
The  effect  of  light  having  a  wave  length  is  such  that 
as  a  general  rule  we  can  get  no  advantage  in  carrying 
the  magnification  above  fifty,  or  one  hundred  at  the 


54         ASTRONOMICAL    INSTRUMENTS 

most,  for  each  inch  of  aperture.  That  is  to  say,  with  a 
three-inch  telescope  we  should  gain  no  advantage  by 
using  a  power  much  above  one  hundred  and  fifty,  and 
certainly  none  above  three  hundred. 

But  a  large  telescope  also  has  its  defects,  owing  to 
the  impossibility  of  bringing  all  the  light  to  absolutely 
the  same  focus.  There  is  a  limit  to  the  magnification 
which  can  be  used,  rather  difficult  to  define  exactly,  but 
of  which  the  observer  will  be  very  sensible  when  he  looks 
into  the  instrument  and  sees  the  blue  aureole  already 
mentioned. 

But  there  is  still  another  trouble,  which  annoys  the 
astronomer  more  than  all  others,  but  which  the  public 
rarely  understands. 

We  see  a  heavenly  body  through  a  thickness  of  atmos- 
phere which,  were  it  all  compressed  to  the  density  that 
it  has  around  us,  would  be  equal  to  about  six  miles.  We 
know  that  when  we  look  at  a  body  six  miles  away,  we 
see  its  outlines  softened  and  blurred.  This  is  mainly 
because  the  atmosphere  through  which  the  rays  have  to 
pass  is  constantly  in  motion,  thus  producing  an  irregular 
refraction  which  makes  the  body  look  wavy  and  tremu- 
lous. The  softened  and  blurred  effect  thus  produced  is 
magnified  in  a  telescope  as  many  times  as  the  object 
itself.  The  result  is  that  as  we  increase  the  magnify- 
ing power  we  increase  a  certain  indistinctness  in  the 
vision  in  the  same  proportion.  The  amount  of  this 
indistinctness  depends  very  much  on  the  condition  of 
the  air.  The  astronomer  having  this  in  mind  tries  to 
find  a  perfectly  clear  air,  or,  rather,  air  which  is  very 


MOUNTING    OF    THE    TELESCOPE       55 

steady,  so  that  the  heavenly  bodies  will  look  sharp  when 
seen  through  it. 

We  frequently  see  calculations  showing  how  near  the 
moon  can  be  brought  to  us  by  using  some  high  magnify- 
ing power.  For  example,  with  a  power  of  one  thou- 
sand we  see  it  as  if  it  were  two  hundred  and  forty  miles 
away ;  with  about  five  thousand,  as  if  it  were  forty-eight 
miles  away.  This  calculation  is  quite  correct  so  far  as 
the  apparent  size  of  any  object  on  the  moon  is  concerned, 
but  it  takes  no  account  either  of  the  imperfections  of  the 
telescope  or  the  bad  effect  produced  by  the  atmosphere. 
The  result  of  both  of  these  defects  is  that  such  calcula- 
tions do  not  give  a  correct  idea  of  the  truth.  I  doubv 
whether  any  astronomer  with  any  telescope  now  in  exist- 
ence could  gain  a  great  advantage,  in  the  study  of  such 
an  object  as  the  moon  or  a  planet,  by  carrying  his  mag- 
nification above  a  thousand,  unless  on  very  rare  occa- 
sions in  an  atmosphere  of  unusual  stillness. 

Mounting  of  the  Telescope 

Those  who  have  never  used  a  telescope  are  apt  to  think 
that  the  work  of  observing  with  it  is  simply  to  point  it  at 
a  heavenly  body  and  examine  the  latter  through  it.* 


*  The  writer  recalls  that  when  Mr.  James  Lick  was  founding 
the  observatory  which  has  since  become  so  celebrated,  the  great 
telescope  was  the  only  feature  which  seemed  to  interest  him,  and 
his  plan  was  to  devote  nearly  all  the  funds  to  making  the  largest 
lens  possible.  He  did  not  see  why  such  a  complicated  instrument 
as  that  used  by  astronomers  was  necessary.  The  troublesome  prob- 
lem of  seeing  a  heavenly  body  through  a  telescope  had  to  be  ex- 
plained to  him. 


56          ASTRONOMICAL    INSTRUMENTS 

But  let  us  try  the  experiment  of  pointing  a  great  tele- 
scope at  a  star.  A  result  which  perhaps  we  have  not 
thought  of  would  be  immediately  presented  to  our  sight. 
The  star,  instead  of  remaining  in  the  field  of  view*  of 
the  telescope,  very  soon  passes  out  of  it  by  the  diurnal 
motion.  This  is  because,  as  the  earth  revolves  on  its  axis, 
the  star  seems  to  move  in  the  opposite  direction.  This  mo- 
tion is  multiplied  as  many  times  as  the  telescope  magni- 
fies. With  a  high  power,  the  star  is  out  of  the  field 
before  we  have  time  to  examine  it. 

Then  it  must  also  be  remembered  that  the  field  of  view 
is  also  magnified  in  the  same  way,  so  that  it  is  smaller 
than  it  appears,  in  proportion  to  the  magnifying  power. 
For  example,  if  a  magnification  of  one  thousand  be  used, 
the  field  of  view  of  an  ordinary  telescope  would  be  about 
two  minutes  in  angular  measure,  a  patch  of  the  sky  so 
small  that  to  the  naked  eye  it  would  look  like  a  mere 
point.  It  would  be  as  if  we  were  looking  at  a  star 
through  a  hole  one  eighth  of  an  inch  in  diameter  in  the 
roof  of  a  house  eighteen  feet  high.  If  we  imagine  our- 
selves looking  through  such  a  hole  and  trying  to  see  a 
star  we  shall  readily  realise  how  difficult  will  be  the 
problem  of  finding  it  and  of  following  it  in  its  motion. 

This  difficulty  is  overcome  by  a  suitable  mounting  of 
the  telescope,  so  as  to  turn  on  two  axes,  at  right  angles  to 
each  other.  By  the  mounting'  is  meant  the  whole  system 
of  machinery  by  the  aid  of  which  a  telescope  is  pointed 


*By  this   term  is  meant  the  small   circular  patch  of  the  sky 
which  we  see  by  looking  into  the  telescope. 


MOUNTING    OF    THE    TELESCOPE        57 

at  a  star  and  made  to  follow  it  in  its  diurnal  motion.  In 
order  not  to  distract  the  attention  of  the  reader  by  be- 
ginning a  study  of  the  instrument  with  a  view  of  all  the 
details,  we  first  give  an  outline,  showing  the  relation  of 
the  axes  on  which  the  telescope  turns.  The  principal 
axis,  called  the  polar  axis,  is  adjusted  so  as  to  be  parallel 
to  the  axis  of  the  earth,  and  therefore  to  point  at  the 
celestial  pole.  Then,  as  the  earth  turns  from  west  to- 
ward east,  a  clockwork  connected  with  this  axis  turns  the 


FIG.  11. — Axes  on  which  a  Telescope  turns. 

^instrument  frcm  east  toward  west,  with  an  equal  motion. 
Thus  the  rotation  of  the  earth  is  neutralized,  as  it  were, 
by  the  corresponding  rotation  of  the  telescope  in  the 
opposite  direction.  When  the  instrument  is  pointed  at 
a  star  and  the  clockwork  set  going,  the  star  when  once 
found  will  remain  in  the  field  of  view. 

In  order  that  a  telescope  may  be  directed  at  any  point 
of  the  heavens  at  pleasure,  there  must  be  another  axis, 
at  right  angles  to  the  polar  axis.  This  is  called  the 


58         ASTRONOMICAL    INSTRUMENTS 

declination  axis.  It  passes  through  a  sheath  fixed  to  the 
upper  end  of  the  polar  axis  so  as  to  form  a  cross  like 
the  letter  T.  By  turning  the  telescope  on  the  two  axes, 
it  can  be  pointed  wherever  we  choose. 

Owing  to  the  polar  axis  being  parallel  to  that  of  the 
earth,  its  inclination  to  the  horizon  is  equal  to  the  lati- 
tude of  the  place.  In  our  latitudes,  especially  in  the 
southern  portions  of  the  United  States,  it  will  be  nearer 
horizontal  than  vertical.  But  in  the  observatories  of 
northern  Europe,  it  is  more  nearly  vertical. 

It  will  be  seen  that  the  contrivance  we  have  described 
does  not  solve  the  problem  of  bringing  a  star  into  the 
field  of  view  of  the  telescope,  or  as  we  commonly  say, 
of  finding  it.  We  might  grope  round  for  minutes  or 
even  hours  without  succeeding  in  this.  There  are  two 
processes  by  which  a  star  may  be  found : 

Every  telescope  for  astronomical  purposes  is  supplied 
with  a  smaller  telescope  fastened  to  the  lower  end  of  its 
tube,  and  called  the  -finder.  This  finder  is  of  low  magni- 
fying power,  and  therefore  has  a  large  field  of  view. 
By  sighting  along  the  outside  of  it,  the  observer,  if  he 
can  see  the  star,  can  point  the  finder  at  it  so  nearly  that 
it  will  be  in  the  field  of  view  of  the  latter.  Having  found 
it  there,  he  moves  the  telescope  so  that  the  object  shall 
be  seen  in  the  centre  of  the  field.  Having  brought  it 
there,  it  is  in  the  field  of  view  of  the  main  telescope. 

But  most  of  the  objects  which  the  astronomer  has  to 
observe  are  totally  invisible  to  the  naked  eye.  He  must, 
therefore,  have  a  system  by  which  a  telescope  can  be 
pointed  at  a  star,  without  any  attempt  on  his  part  to  see 


THE  MAKING  OF   TELESCOPES          59 

the  latter.  This  is  done  by  graduated  circles,  one  of 
which  is  attached  to  each  axis.  One  of  these  circles  has 
degrees  and  fractions  of  a  degree  marked  upon  it,  so  as 
to  show  the  declination  of  that  point  in  the  heavens  at 
which  the  telescope  is  pointed.  The  other,  attached  to 
the  polar  axis,  and  called  the  hour  circle,  is  divided  into 
twenty-four  hours,  and  these  again  into  sixty  minutes 
each.  When  the  astronomer  wishes  to  find  a  star,  he 
simply  looks  at  the  sidereal  clock,  subtracts  the  right 
ascension  of  the  star  from  the  sidereal  time,  and  thus 
gets  its  "hour  angle"  at  the  moment,  or  its  distance  east 
or  west  of  the  meridian.  He  sets  the  declination  circle 
at  the  declination  of  the  star,  that  is,  he  turns  the  tele- 
scope until  the  degree  on  the  circle  seen  through  a  mag- 
nifying aparatus  is  equal  to  the  declination  of  the  star ; 
and  then  he  turns  the  instrument  on  the  polar  axis  until 
the  hour  circle  reads  its  hour  angle.  Then,  starting  his 
clockwork,  he  has  only  to  look  into  the  telescope  and 
there  is  the  object. 

If  all  this  seems  a  complicated  operation  to  the  reader, 
he  has  only  to  visit  an  observatory  and  see  how  simply  it 
is  all  done.  He  may  thus  in  a  few  minutes  gain  a  practi- 
cal idea  of  sidereal  time,  hour  angle,  declination,  etc.v 
which  will  make  the  whole  subject  much  clearer  than  any 
mere  description. 

The  Making  of  Telescopes 

Let  us  return  to  some  interesting  matters,  mostly  his- 
torical, connected  with  the  making  of  telescopes.  The 
great  difficulty,  which  requires  special  native  skill  of  the 


60         ASTRONOMICAL    INSTRUMENTS 

rarest  kind,  is,  as  we  have  already  intimated,  that  of  con- 
structing the  object-glass.  The  slightest  deviation  from 
the  proper  form — a  defect  consisting  in  some  part  of 
the  object-glass  being  too  thin  by  a  hundred  thousandth 
part  of  an  inch — would  spoil  the  image. 

The  skill  of  the  optician  who  figures  the  glass,  that  is 
to  say,  who  polishes  it  into  the  proper  shape,  is  by  no 
means  all  that  is  required.  The  making  of  large  disks 
of  glass  of  the  necessary  uniformity  and  purity  is  a 
practical  problem  of  equal  difficulty.  Any  deviation  from 
perfect  uniformity  in  the  glass  will  be  as  injurious  to  its 
performance  as  a  defect  in  its  figure.* 

A  century  ago  it  was  found  especially  difficult  to  mako 
flint  glass  of  the  necessary  uniformity.  This  substance 
contains  a  considerable  amount  of  lead,  which,  during 
the  process  of  melting  the  glass,  would  sink  toward  the 
bottom  of  the  pot,  thus  making  the  bottom  portion  of 
greater  refracting  power  than  the  upper  portion.  The 
result  was  that,  at  that  time,  a  telescope  of  four  or  five 
inches  aperture  was  considered  of  great  size.  Quite  early 
in  the  centurj^,  Guinand,  a  Swiss,  found  a  process  by 
which  larger  disks  of  flint  glass  could  be  made.  He  pro- 
fessed to  have  some  secret  process  of  doing  this,  but  there 
is  some  reason  to  believe  that  his  secret  consisted  only  in 
the  constant  and  vigorous  stirring  of  the  melted  glass 

*  It  is  frequently  proposed  by  persons  not  acquainted  with  the 
delicate  points  of  the  problem  to  make  a  telescope  of  large  size  by 
putting  together  different  pieces  of  glass,  each  of  the  proper  shape, 
to  form  a  lens.  The  idea,  ingenious  though  it  looks,  is  thor- 
oughly impracticable,  for  the  simple  reason  that  it  is  impossible  to 
make  two  pieces  of  glass  of  exactly  the  same  refracting  power. 


ALVAN    CLARK    AND    HIS    GENIUS       61 

while  it  was  being  fused  in  the  pot.  However  this  may 
have  been,  he  succeeded  in  making  disks  of  larger  and 
larger  size. 

To  utilize  these  disks  required  an  optician  of  corre- 
sponding skill  to  grind  and  polish  them  into  proper 
shape.  Such  an  artist  was  found  in  the  person  of  Fraun- 
hofer,  of  Munich,  who,  about  1820,  made  telescopes  as 
large  as  nine  inches  aperture.  He  did  not  stop  here,  but, 
about  1840,  succeeded  in  making  two  objectives,  each  of 
fourteen  German  inches,  or  about  fifteen  English  inches 
in  diameter.  These,  far  exceeding  any  before  made,  were 
at  the  time  regarded  as  marvellous.  One  of  these  instru- 
ments was  acquired  by  the  Pulkova  Observatory  in  Rus- 
sia ;  the  other  was  acquired  by  the  Harvard  Observatory 
at  Cambridge,  Mass.  The  latter,  after  a  lapse  of  more 
than  half  a  century,  is  still  in  efficient  use. 

Alvan  Clark  and  His  Genius 

After  Fraunhofer's  death  it  was  doubtful  whether  his 
skill  had  died  with  him,  or  had  passed  to  a  successor. 
The  latter  appeared  where  none  would  have  thought  of 
looking  for  him,  in  the  person  of  an  obscure  portrait 
painter  of  Cambridgeport,  Mass.,  named  Alvan  Clark. 
The  fact  that  such  a  man,  with  scarcely  the  elements 
of  technical  education  and  without  training  in  the  use 
of  optical  instruments,  should  have  done  what  he  did, 
illustrates  in  a  striking  way  what  an  important  element 
native  talent  is  in  such  a  case.  He  seemed  to  have  an 
intuitive  conception  of  the  nature  of  the  problem, 
coupled  with  extraordinary  acuteness  of  vision  in  solving 


62          ASTRONOMICAL    INSTRUMENTS 

it.  Moved  by  that  irrepressible  impulse  which  is  a  mark 
of  genius,  he  purchased  in  Europe  the  rough  disks  of 
optical  glass  necessary  to  make  small  telescopes.  Having 
succeeded  in  making  one  of  four  inches  aperture  to  his 
satisfaction,  the  problem  was  to  make  his  skill  known  to 
astronomers.  I  regret  to  say  that  he  found  this  a  very 
difficult  part  of  his  task.  The  director  of  the  Harvard 
Observatory  would  not  believe  that  Mr.  Clark  could  make 
a  really  good  telescope.  When  the  optician  took  his 
first  instrument  up  to  the  observatory  to  be  tested,  the 
astronomer  called  his  attention  to  the  fact  that  it  showed 
a  little  tail  attached  to  the  star,  which,  of  course,  had  no 
real  existence,  and  was  supposed  to  arise  from  a  serious 
defect  in  the  figure  of  the  glass.  Mr.  Clark  saw  it,  but 
was  sure  it  had  not  been  there  before.  He  could  not  ex- 
plain it  at  the  time,  but  afterwards  found  that  it  was 
caused  by  the  unequal  temperature  of  the  air  in  the  tube 
of  the  telescope  when  it  was  exposed  under  the  sky  at 
night. 

Unable  to  secure  any  effective  recognition  at  home, 
he  determined  to  try  abroad.  He  made  a  larger  instru- 
ment, scanned  the  heavens  with  it  and  discovered  several 
close  and  difficult  double  stars.  He  wrote  out  descrip- 
tions of  these  objects  and  sent  them  to  Rev.  W.  R. 
Dawes,  an  amateur  astronomer  in  England,  devoted  to 
this  branch  of  the  science.  Mr.  Dawes  was  a  lovely  char- 
acter. He  looked  at  the  objects  described  by  Clark  and 
found  great  difficulty  in  making  them  out.  Yet  the  de- 
scriptions were  so  accurate  that  it  was  evident  to  him 
that  Mr.  Clark's  instrument  must  be  of  the  highest  class. 


CLARK'S    GREAT    TELESCOPES  63 

He  wrote  asking  him  to  look  at  some  other  objects  and 
describe  them.  When  the  description  was  received  it  was 
found  to  be  exact.  No  doubt  could  remain.  The  result 
was  a  further  correspondence,  the  purchase  by  Mr. 
Dawes  of  the  largest  and  best  instrument  that  Mr.  Clark 
could  then  make,  and  a  friendship  which  continued  as 
long  as  Mr.  Dawes  lived. 

Mr.  Clark  now  secured  recognition  in  his  own  country 
and  became  ambitious  to  make  the  largest  refracting 
telescope  that  had  ever  been  known.  This  was  one  of 
eighteen  inches  diameter,  which  was  completed  about 
1860  for  the  University  of  Mississippi.  While  testing 
it  at  his  workshop,  a  discovery  of  a  most  interesting 
character  was  made  with  it  by  Mr.  George  B.  Clark,  the 
son.  This  was  a  companion  of  Sirius,  which  had  been 
known  to  exist  by  its  attraction  on  Sirius,  but  had  never 
been  seen  by  human  eye.  The  breaking  out  of  the  Civil 
War  prevented  the  University  of  Mississippi  from  tak- 
ing the  telescope,  and  the  latter  was  acquired  by  citizens 
of  Chicago.  It  is  now  mounted  at  the  Northwestern 
University  in  Evanston,  111. 

The  making  of  disks  of  glass  of  larger  and  larger 
size  was  continued  by  the  great  glass  works  of  Chance 
&  Company,  in  England.  But  they  found  the  work 
too  delicate  and  too  troublesome,  and  allowed  it  to  pass 
into  the  hands  of  Feil  of  Paris,  son-in-law  of  Guinand. 
With  the  glass  supplied  by  these  two  parties,  Mr.  Clark 
made  larger  and  larger  telescopes.  First  was  the  twenty- 
six-inch  telescope  for  the  Naval  Observatory  at  Wash- 
ington and  a  similar  one  for  the  University  of  Virginia. 


64          ASTRONOMICAL    INSTRUMENTS 

Then  followed  a  still  larger  instrument,  thirty  inches  in 
diameter,  for  the  Observatory  of  Pulkova,  Russia.  Next 
was  completed  the  thirty-six-inch  instrument  of  the  Lick 
Observatory,  which  has  done  such  splendid  work. 

After  the  death  of  Feil,  the  business  was  taken  up  by 
Mahtois,  who  made  optical  glass  of  a  purity  and  uni- 
formity that  no  one  before  him  had  ever  approached. 
He  furnished  the  disks  with  which  the  Clarks  figured 
the  objective  for  the  Yerkes  telescope  of  the  University 
of  Chicago.  This  is  about  forty  inches  in  diameter,  and 
is  the  largest  refracting  telescope  now  in  actual  use  for 
astronomical  purposes. 

Our  readers  have  doubtless  been  interested  in  the  great 
telescope  of  the  Paris  Exposition  of  1900,  which  is  yet 
larger  than  that  of  Chicago,  being  of  forty-seven  inches 
aperture.  This  instrument  is  of  such  immense  size  that 
it  cannot  be  mounted  and  pointed  at  the  heavens  in  the 
usual  way.  It  is  therefore  fixed  in  a  horizontal,  north 
and  south  position,  and  the  rays  of  the  object  to  be  ob- 
served are  reflected  into  it  by  an  immense  plane  mirror. 
The  question  whether  this  contrivance  has  been  success- 
ful with  so  large  an  instrument  is  one  that  is  not  yet 
settled  with  astronomical  precision.  Nothing  has  yet 
been  done  with  this  instrument,  which,  it  is  feared,  is 
so  imperfect  in  make  as  to  serve  no  better  purpose  than 
that  of  a  toy. 

The  engineering  problem  of  mounting  a  great  tele- 
scope is  by  no  means  a  simple  one.  It  was  one  in  which 
Mr.  Clark  was  less  successful  than  in  the  construction  of 
his  object-glasses.  In  the  case  of  the  later  telescopes  the 


GREAT   TELESCOPES 


65 


FiQ.  12.— Great  Telescope  of  the  Yerkes  Observatory, 

Swazey. 


Warner 


66         ASTRONOMICAL    INSTRUMENTS 

mountings  of  the  great  instruments  were  made  by  other 
parties.  That  of  the  Pulkova  telescope  was  made  by  the 
Repsolds  of  Hamburg,  the  most  noted  makers  of  fine 
astronomical  instruments  in  Europe.  The  Lick  and 
Chicago  telescopes  were  mounted  by  Warner  &  Swazey, 
of  Cleveland,  Ohio,  who  are  gaining  the  highest  reputa- 
tion in  this  class  of  work.  In  the  case  of  the  Chicago 
telescope,  arrangements  were  devised  by  them  which  sur- 
pass all  ever  before  thought  of.  The  observer  has  only 
to  touch  electric  buttons  to  have  all  the  work  of  pointing 
and  moving  the  telescope  performed  by  electricity. 


n 

THE  REFLECTING   TELESCOPE 

ALTHOUGH  the  refracting  telescope  is  that  in  most 
general  use,  there  is  another  form  of  instrument  of  radi- 
cally different  construction.  Its  main  feature  is  that 
the  functions  of  the  object-glass  are  performed  by  a 
slightly  concave  mirror.  That  such  a  mirror  reflects 
parallel  rays  falling  upon  it  to  a  focus,  is  doubtless  well 
known  to  our  readers.  The  focus  is  situated  about  half- 
way between  the  mirror  and  its  centre  of  curvature. 

This  form  of  instrument  has  an  enormous  advantage 
in  its  freedom  from  the  "secondary  aberration"  which 
we  have  already  described  as  inherent  in  the  refracting 
telescope.  Another  advantage  which  it  possesses  is  that 
it  can  be  made  of  larger  dimensions  than  the  other.  The 
extreme  limit  so  far  reached  in  the  refractor,  as  we  have 
already  stated,  is  four  feet „  but  the  forty-inch  aperture 
-of  the  Yerkes  telescope  is,  up  to  the  present  time,  the 
limit  in  actual  use  for  astronomical  research.  But,  more 
than  half  a  century  ago9  Lord  Rosse  constructed  his 
great  reflector  of  six  feet  diameter.  Judging  by  its 
size  alone,  this  instrument  ought  to  give  several  times 
more  light,  and  therefore  show  far  minuter  stars,  than 
any  refracting  telescope  yet  made.  But,  for  some  rea- 
son, its  performance — and,  indeed,  that  of  reflectors 
generally — has  not  corresponded  to  the  size. 


68         ASTRONOMICAL    INSTRUMENTS 

The  practical  difficulties  in  using  a  reflector  are  several 
in  number.  The  first  and  most  obvious  one  is  that  the 
rays  are  reflected  back  in  the  direction  from  which  they 
came.  To  see  the  image  the  observer  must  look  into  the 
mirror  as  it  were.  If  he  does  this  directly,  his  head  and 
shoulders  will  cut  off  the  light  that  falls  on  at  least  the 
central  regions  of  the  mirror.  Some  contrivance  for  re- 
flecting this  light  away  is  therefore  necessary.  Two 
ways  of  doing  this  are  in  use.  In  what  is  known  as  the 
Cassegranian  reflector,  a  smaller,  slightly  convex  mirror 
is  interposed  between  the  focus  and  the  principal  mirror. 
An  opening  is  made  in  the  centre  of  the  latter,  through 
which  the  rays  are  reflected  back  by  the  smaller  mirror. 
The  curvature  and  positions  of  the  two  are  so  adjusted 
that  the  image  of  the  distant  object  shall  be  formed  in 
this  opening.  The  only  telescope  of  this  kind  in  actual 
use  is  the  great  Melbourne  reflector,  of  four  feet  diam- 
eter, made  by  Sir  Howard  Grubb,  of  Dublin. 

The  contrivance  most  in  use  was  designed  by  Sir  Isaac 
Newton.  It  consists  of  a  diagonal  reflector,  which  may 
be  a  mere  glass  prism,  placed  just  inside  the  focus.  Its 
reflecting  surface  makes  an  angle  of  forty-five  degrees 
with  the  axis  of  the  telescope,  and  therefore  reflects  the 
rays  laterally  to  the  side  of  the  tube.  Here  they  are 
observed  with  an  ordinary  eyepiece.  This  instrument  is 
known  as  the  Newtonian  reflector. 

It  is  remarkable  that,  notwithstanding  the  immense 
improvement  in  the  mechanical  processes  necessary  in 
constructing  and  mounting  a  reflecting  telescope,  no  at- 
tempt has  ever  been  made  to  even  equal  Lord  Rosse's 


FlG.  lo. — Swtiuii  of  a  Newtonian  Reflecting  Telescopy 


70          ASTRONOMICAL    INSTRUMENTS 

great  instrument  in  dimensions.  The  largest  mirrors 
so  far  successfully  made  and  used  have  been  about  four 
feet  in  diameter.  About  fifty  years  ago,  Mr.  Lassell 
made  one  of  this  size,  with  which  he  discovered  two 
new  satellites  of  Uranus.  More  recently,  Mr.  A.  A. 
Common,  F.R.S.,  has  constructed  a  mirror  of  the  same 
size.  This  has  been  used  in  taking  photographs  of 
nebulae  and  other  faint  objects,  for  which  this  form  of 
telescope  seems  well  designed. 

The  great  difficulty  in  using  a  large  mirror  is  that  it 
bends  under  the  influence  of  its  own  weight.  It  would 
seem  that  when  the  diameter  exceeds  four  feet,  no  way 
of  completely  avoiding  this  difficulty  has  yet  been  put 
into  successful  use.  A  mirror  of  five  feet  diameter  is, 
however,  being  made  at  the  Yerkes  Observatory  by  Mr. 
Ritchie,  in  which,  it  is  hoped,  all  the  difficulties  will  be 
surmountedo 

In  the  instruments  of  Lord  Rosse  and  Mr.  Lassell,  the 
mirror  was  made  of  an  alloy,  known  as  speculum  metal. 
Recently,  however,  the  use  of  speculum  metal  has  been 
superseded  by  another  arrangement.  The  concave  mir- 
ror is  made  of  a  large  disk  of  glass,  which  is  ground  and 
polished  into  nearly  spherical  form,  or  to  speak  more  ac- 
curately, a  parabolic  form,  because  the  latter  is  necessary 
to  bring  all  the  rays  to  one  focus.  A  thin  coating  of 
silver  is  then  deposited  on  the  surface  of  the  glass,  which 
is  susceptible  of  a  high  polish,  and  reflects  more  light 
than  polished  metal. 


in 

THE  PHOTOGRAPHIC  TELESCOPE 

ONE  of  the  greatest  advances  in  practical  astronomy 
in  our  time  has  been  brought  about  by  photographing 
the  heavenly  bodies.  This  is  so  simple  a  process  that  the 
slowness  of  its  introduction  may  seem  curious.  Back  in 
the  early  '40's,  Professor  Draper,  of  New  York,  the  well- 
known  chemist,  succeeded  in  making  a  daguerreotype 
of  the  moon0  When  the  system  of  photography  by  our 
present  process  on  a  glass  negative  was  invented,  Pro- 
fessor Bond,  of  the  Harvard  Observatory,  and  Mr.  L.  M. 
Kutherfurd,  an  eminent  astronomer  of  New  York,  both 
began  to  apply  the  art  to  the  moon  and  stars.  Mr. 
Rutherfurd  brought  his  work  to  such  perfection  that  his 
photographs  of  the  Pleiades  and  other  clusters  of  stars 
are  still  of  great  value  in  astronomy. 

A  photograph  of  the  stars  can  be  made  by  an  ordinary 
^camera  if  we  only  mount  it  like  an  equatorial  telescope 
so  that  it  shall  follow  the  star  in  its  diurnal  motion.  A 
very  few  minutes  exposure  will  suffice  to  take  a  picture 
of  more  stars  than  can  be  seen  by  the  naked  eye ;  in  fact, 
with  a  large  camera,  this  will  not  require  a  minute.  But 
what  is  generally  used  by  the  astronomer  is  a  photo- 
graphic telescope.  Any  ordinary  telescope  will  serve 
the  purpose,  but  in  order  to  get  the  best  results  the 
object-glass  of  the  telescope  must  be  especially  made  to 


72          ASTRONOMICAL    INSTRUMENTS 

bring  to  a  focus  those  rays  of  light  to  which  the  photo- 
graphic film  is  most  sensitive.  So  rapid  has  been  the 
progress  during  the  past  few  years  that  the  greater  part 
of  the  astronomical  work  of  the  future  seems  likely  to  be 
done  by  photography.  The  great  advantage  of  the 
method  is  that  when  a  picture  either  of  some  heavenly 
body  or  of  the  stars  in  the  sky  is  taken,  it  can  be  studied 
and  measured  at  leisure  with  all  the  care  the  astronomer 
chooses  to  bestow  upon  it,  while  the  observation  in  the 
heavens  is  nearly  always  more  or  less  hurried,  and  made 
difficult  by  the  diurnal  motion  of  the  star. 

Formerly  the  spots  on  the  sun  were  investigated  by 
watching  that  luminary  through  the  telescope,  recording 
the  number  of  spots,  and  measuring  their  position  on  the 
solar  disk.  Now,  at  the  Greenwich  Observatory  and  else- 
where, a  photograph  of  the  sun  is  taken  almost  every 
day,  and  the  position  of  the  spots  is  found  by  measuring 
the  photograph.  Thus  a  study  of  the  sun  and  the 
changes  going  on  on  its  surface  is  kept  up  from  year  to 
year. 

Formerly  the  astronomer  studied  the  physical  con- 
stitution of  a  comet  by  making  a  drawing  of  it.  This 
was  a  rather  uncertain  process,  and  as  a  general  rule  no 
two  men  would  quite  agree  in  the  minute  details.  Now 
the  comet  is  photographed  and  a  study  is  made  upon  the 
negative.  The  same  remark  applies  to  nebulae.  Draw- 
ings of  them  are  no  longer  made — only  photographs 
which  show  a  great  deal  more  than  any  drawing  will. 


IV 

THE  SPECTROSCOPE 

THE  spectroscope  is  an  instrument  for  analysing 
light.  It  is  a  much  more  recent  instrument  than  the 
telescope,  having  first  been  applied  to  astronomical  ob- 
servation about  1864.  To  convey  an  intelligent  idea 
of  its  use  we  must  say  something  about  the  heat  and  light 
radiated  by  the  heavenly  bodies. 

We  know  that  the  sun,  a  gas  light,  or  other  bright 
body  gives  us  heat  as  well  as  light.  A  very  simple  obser- 
vation will  show  that  the  rays  of  heat  proceed  in  straight 
lines  like  those  of  light,  and  that  they  can  pass  through 
air  and  other  transparent  bodies  without  warming  them, 
just  as  light  does.  If  we  make  a  large  fire  on  the  hearth 
in  a  perfectly  cold  room,  we  shall  feel  the  heat  on  our 
faces  although  the  air  may  be  frosty.  A  striking  experi- 
ment is  that  of  making  a  lens  out  of  ice  and  using  it  as 
a  burning  glass.  The  rays  of  the  sun  passing  through 
the  ice  may  be  concentrated  so  as  to  burn  the  hand,  and 
that  without  the  ice  melting. 

It  was  formerly  supposed  that  heat  and  light  were  two 
distinct  agents ;  now  it  is  known  that  such  is  not  the  case. 
As  emitted  by  a  hot  body  both  may  be  called  by  the 
general  name  of  radiance.  All  radiance,  when  it  falls 
on  a  surface,  produces  heat,  just  as  the  blaze  of  the  fire 
produces  heat  on  the  walls  of  a  room.  But  not  all  radi- 


74          ASTRONOMICAL    INSTRUMENTS 

ance  affects  the  optic  nerve  of  the  eye  so  as  to  produce  a 
sensation  of  light  and  enable  us  to  see  bodies. 

It  is  now  know  that  radiance  consists  of  something  in 
the  nature  of  waves  in  an  ethereal  medium  which  fills  all 
space,  even  to  the  most  distant  star.  These  waves  are 
exceedingly  short.  To  form  an  idea  of  their  length  we 
must  measure  by  the  micron,  which  is  one  thousandth 
of  a  millimetre.  Those  which  produce  the  sensation  of 
light  on  the  optic  nerve  mostly  range  between  four  and 
seven  tenths  of  a  micron.  This  allows  between  forty  and 

eighty  thousand  waves 
'~~\^^-~^^~l~~*^^^^~T~'  to  the  inch.     We  rep- 

iwAVE-LENGTHS;  -1  i 

•  i  resent  these   waves   by 

FIG,  14. —  Wave  Length  of  Light.          the  little  wave  line  in 

the  figure.  The  dis- 
tance between  the  dotted  lines  is  the  wave  lengths.  The 
peculiar  feature  of  the  radiance  emitted  by  the  sun,  or 
any  other  body  that  is  not  transparent,  is  that  it  is  not 
all  of  the  same  wave  length,  but  of  a  very  wide  range 
of  wave  lengths  all  mixed  together.  We  must  imagine 
that  between  the  rays  which  we  represent  in  the  figure 
there  are  an  infinity  of  others,  all  varying  in  their  wave 
lengths.  In  this  respect  radiance  is  like  the  waves  of 
the  ocean,  which  range  in  length  from  several  hundred 
yards  to  a  few  inches,  all  piled  upon  each  other. 

When  the  radiance  passes  through  a  glass  prism  it  is 
refracted  from  its  course.  Different  wave  lengths  are 
refracted  differently,  but  waves  of  the  same  length  are 
always  refracted  by  the  same  amount.  This  is  shown  by 
the  familiar  experiment  of  forming  a  spectrum  of  the 


THE    SPECTROSCOPE 


75 


sun  with  a  triangular  prism.  Arranging  the  light  to 
be  thrown  on  a  screen,  we  see  red  light  at  the  bottom, 
then  yellow  above  it,  then  in  succession,  green,  blue,  and 
violet.  This  arrangement  of  colours 
on  a  surface  is  called  a  spectrum.  The 
colour  of  the  light  in  the  spectrum 
depends  on  the  wave  length.  If  the 
wave  length  is  greater  than  about 
seventy-five  one-hundredths  of  a  mi- 
cron, that  is,  one  forty-four-thou- 
sandth of  an  inch,  the  eye  does  not 
see  it,  and,  for  us,  it  passes  simply  as 
heat.  From  this  length  to  one  fifty- 
thousandth  it  looks  red,  when  a  little 
shorter  it  looks  scarlet,  then  yellow, 
and  so  on.  Shorter  than  forty -three 
one-hundredths  of  a  micron  it  is  diffi- 
cult to  see  it  at  all.  But  the  violet 
light  affects  the  photographic  plate 
even  more  strongly  than  the  light 
which  looks  brightest  to  the  eye.  The 
light  which  is  most  easily  photo- 
graphed is  the  blue  and  violet,  and  as 
we  go  toward  the  red  the  photo- 
graphic effect  diminishes. 

All  bodies   emit  radiance,   but,   at 
ordinary     temperatures,     the 


FIG.  15.  —  Arrange- 
ment of  the  Colours 
of  the  Spectrum, 
with  the  Dark  Linen 
A,£,  C,D,etc.,of 
the  Spectrum. 


wave 


lengths  of  this  radiance  are  too  long  to  be  visible 
to  the  eye.  Not  until  we  heat  a  body  red  hot 
does  it  emit  radiance  of  wave  length  short  enough  to 


76          ASTRONOMICAL    INSTRUMENTS 

form  light.  As  we  make  it  hotter  it  still  emits  more  and 
more  waves  of  long  wave  lengths,  and  also  waves  of 
shorter  and  shorter  wave  lengths.  Thus  as  we  heat  up 
a  piece  of  iron,  it  appears  first  as  red  hot,  and  afterward 
as  white  hot. 

The  possibility  of  reaching  conclusions  about  the  con- 
stitution of  a  hot  body  from  the  light  which  it  emits  arises 
from  the  fact  that  different  bodies  emit  light  of  different 
wave  lengths.  If  the  body  is  solid,  it  emits  light  of  all 
wave  lengths,  and  we  cannot  tell  much  about  it.  But  if 
it  is  a  mass  of  transparent  gas,  it  only  emits  light  of  cer- 
tain wave  lengths,  depending  on  the  nature  of  the  gas. 

The  easiest  way  of  making  a  gas  emit  its  peculiar 
light  is  by  passing  an  electric  spark  or  current  through 
it.  Then,  if  we  analyse  the  light  produced  by  the  spark 
with  a  prism,  we  find  that  the  spectrum  is  composed  of 
one  or  more  bright  lines,  varying  in  position  according 
to  the  nature  of  the  gas.  Thus  we  have  a  spectrum  of 
hydrogen,  another  of  oxygen,  and  others  of  almost  all 
the  bodies  which  we  know.  Solid  bodies,  including  all 
the  metals,  can  be  made  to  give  their  spectrum  by  being 
heated  so  intensely  by  the  electric  spark  that  a  small 
quantity  of  the  body  is  changed  into  a  gas.  Thus  we 
may  even  form  a  spectrum  of  iron,  which  the  practised 
observer  can  immediately  detect  as  iron  by  the  position 
and  arrangement  of  the  lines  of  the  spectrum. 

How  the  Stars  are  Analysed 

The  fundamental  principle  of  spectrum  analysis  is 
that  if  the  light  of  an  incandescent  body  passes  throuHi 


HOW  THE  STARS  ARE  ANALYSED       77 

a  gas  which  is  cooler  than  the  body,  the  latter  will  cull 
out  and  absorb  from  the  light  those  wave  lengths  which 
it  would  emit  if  it  were  itself  incandescent.  The  result 
is  that  the  spectrum  from  the  solid  body  will  be  seen 
crossed  by  certain  dark  lines,  depending  on  the  nature 
cf  the  gas  through  which  the  light  has  passed.  Thus, 
if  we  observe  an  electric  light  through  a  prism  in  its 
immediate  neighbourhood,  the  spectrum  will  be  unbroken 
from  one  end  to  the  other.  But  if  the  light  is  at  a  great 
distance,  we  shall  see  it  crossed  by  a  great  number  of  dark 
lines.  These  lines  are  produced  by  the  air  through  which 
the  light  has  passed  culling  out  the  light  which  has  cer- 
tain wave  lengths.  It  is  of  interest  that  the  aqueous  va- 
pour in  the  air  is  the  most  powerful  agent  in  this,  and 
culls  out  great  groups  of  lines,  by  which  its  presence  in 
the  air  can  be  immediately  detected.  The  darkest  of  the 
lines  found  in  the  spectrum  of  the  sun  are  designated  by 
the  letters  A,  B,  C,  etc.,  as  shown  in  the  preceding 
figure. 

We  may  describe  the  spectroscope  in  the  most  compre- 
hensive way  by  saying  that  it  is  an  instrument  for 
studying  the  spectra  of  bodies,  whether  in  the  heavens 
or  on  the  earth. 

The  studies  of  the  heavenly  bodies  with  the  spectro- 
scope have  two  objects.  One  is  to  determine  the  nature 
of  the  bodies;  the  other  their  motions  to  or  from  us. 
The  possibility  of  the  latter  is  one  of  the  most  wonderful 
achievements  of  modern  science.  If  a  star  is  coming 
toward  us,  the  wave  length  of  the  light  which  it  emits  is 
slightly  shorter  in  consequence  of  the  motion;  if  it  is 


78          ASTRONOMICAL    INSTRUMENTS 

going  away  from  us,  it  is  longer.  Thus,  by  measuring 
the  positions  of  its  lines  in  the  spectrum,  it  is  possible 
to  determine  whether  a  star  is  approaching  us  or 
moving  away  from  us. 

In  recent  years  the  studies  of  the  spectra  of  stars  have 
been  made  almost  entirely  by  photography.  It  is  found 
that,  as  in  other  cases,  the  sensitive  plates  now  used  in 
that  art  will  take  impressions  of  objects  which  the  eye 
cannot  see  in  the  telescope.  So  the  astronomer  photo- 
graphs the  spectrum  of  a  star,  which  will  show  all  the 
lines  he  can  see  with  the  naked  eye,  and  perhaps  a  great 
many  more.  The  positions  of  these  lines  are  measured 
and  studied,  and  the  astronomer's  conclusions  are  drawn 
from  these  studies. 


V 

OTHER  ASTRONOMICAL  INSTRUMENTS 

IT  is  commonly  supposed  that  the  principal  work  of 
an  astronomer  is  to  study  the  stars  as  he  sees  them  in 
his  telescope.  This  is  true  only  in  the  sense  that  a  tele- 
scope is  a  necessary  part  of  almost  every  astronomical 
instrument.  But  the  mere  studying  of  a  star  with  a 
telescope  is  a  very  small  part  of  the  astronomer's  work. 
The  most  important  practical  use  of  astronomy  to  our 
race  consists  in  the  determination  of  the  latitudes  and 
longitudes  of  points  on  the  earth's  surface,  so  that  we 
may  know  where  towns  and  cities  are  situated  and  be 
able  to  make  a  map  of  a  state  or  country.  This  re- 
quires a  knowledge  of  the  exact  positions  of  the  stars  in 
the  heavens,  that  is  to  say,  of  their  right  ascension  and 
declination.  We  have  shown  in  a  former  chapter  how 
these  quantities  correspond  to  longitude  and  latitude  on 
the  earth's  surface.  Through  that  correspondence  an 
observer  may  determine  his  latitude  by  the  star's  dec- 
lination and  his  longitude  by  its  right  ascension,  com- 
bined with  a  knowledge  of  the  sidereal  time  at  a  place 
of  known  longitude. 

The  figures  and  dimensions  of  the  planets,  the  motions 
of  the  satellites,  the  orbits  of  planets  and  comets,  the 
structure  of  nebula?  and  clusters  of  stars — all  these  offer 
fields  of  astronomical  investigation  to  which  there  is 


80          ASTRONOMICAL    INSTRUMENTS 

no  end,  and  in  order  to  make  these  investigations  other 
instruments  besides  the  telescope  are  necessary. 

The  Meridian  Circle  and  Clock 

The  problem  which  demands  most  attention  from  the 
v/orking  astronomer  in  an  observatory  is  the  determina- 
tion of  the  positions  of  the  heavenly  bodies.  The  prin- 


FIG.  1C. — A  Meridian  Instrument. 

cipal  instrument  for  making  these  determinations  is  the 
meridian  circle,  called  also  a  meridian  instrument.  This 
consists  of  a  telescope  supported  on  a  horizontal  east  and 
west  axis,  at  right  angles  to  its  length,  so  that  its  line 
of  sight  can  move  only  along  the  meridian.  If  it  points 


MERIDIAN    CIRCLE    AND    CLOCK         81 

exactly  south  you  can  turn  it  on  the  axis  until  the  line 
of  sight  passes  through  the  zenith,  and  still  farther  until 
it  passes  through  the  pole  on  the  north  horizon;  but 
you  cannot  turn  it  east  or  west.  This  might  seem  to  re- 
strict its  usefulness,  but  it  is  on  this  restriction  of  its 
motion  that  its  usefulness  depends.  The  great  value 
of  this  instrument  is  that  it  enables  us  to  determine  the 
right  ascension  of  a  star  without  taking  any  measure- 
ment but  one  of  time.  In  a  former  chapter  we  described 
sidereal  time,  the  units  of  which  are  slightly  shorter 
than  those  of  our  ordinary  time,  so  that  a  sidereal  clock 
gains  about  two  hours  every  month  on  an  ordinary  clock. 
The  sidereal  time  at  which  a  star  crosses  the  meridian  is 
the  same  as  its  right  ascension ;  the  problem  of  determin- 
ing the  latter,  therefore,  is  the  simplest  in  the  world. 
We  start  our  sidereal  clock,  set  it  on  the  exact  sidereal 
time,  point  the  telescope  of  the  meridian  circle  to  various 
stars  as  they  are  about  to  cross  the  meridian,  and  note 
the  exact  moment  at  which  each  star  passes.  In  the 
instrument  the  meridian  is  shown  by  a  very  fine  fibre  or 
spider's  web  fixed  in  the  focus  of  the  telescope.  The 
moment  when  the  image  of  the  star  as  seen  in  the  tele- 
scope crosses  this  spider  line  is  that  of  passing  the  me- 
ridian. The  time  by  the  sidereal  clock  then  shows  the 
star's  right  ascension.  If  the  clock  could  be  set  with 
perfect  exactness  and  the  instrument  revolved  exactly 
in  the  plane  of  the  meridian,  right  ascensions  would  be 
determined  in  the  very  simple  way  we  have  described. 

It  unfortunately  happens,  however,  that  no  clock  can 
be  set  with  such  exactness  as  to  satisfy  the  requirements 


82         ASTRONOMICAL    INSTRUMENTS 

of  the  astronomer,  who  wants  to  know  the  time  down  to 
the  tenth  or  even  to  the  hundredth  of  a  second.  More- 
over, no  meridian  circle  can  have  its  axis  set  so  exactly 
east  and  west  that  the  instrument  shall  not  deviate  a  little 
from  the  meridian.  The  astronomer  must  therefore  make 
allowances  for  the  error  of  his  clock  and  for  the  deviation 
of  his  instrument;  and  these  require  much  careful  ob- 
servation and  calculation.  Even  when  he  does  the  best 
he  can,  a  single  observation  will  always  be  liable  to  little 
errors  which  he  wishes  to  make  as  small  as  possible.  He 
does  this  by  repeatedly  determining  the  position  of  every 
star  which  he  puts  upon  his  list.  He  generally  has  to  be 
satisfied  with  three  or  four  observations  on  the  great 
mass  of  the  stars,  but  on  the  more  important  stars  he 
makes  them  by  scores  or  hundreds. 

To  determine  the  declination  of  a  star,  a  graduated 
circle  is  necessary.  This  consists  of  a  brass  or  steel  circle, 
much  like  a  carriage  wheel,  of  which  the  axis  is  the  same 
as  that  on  which  the  telescope  of  the  meridian  instrument 
turns.  The  circle  is  firmly  attached  to  the  axis  so 
that  it  must  turn  with  the  telescope  as  the  latter  sweeps 
along  the  celestial  meridian.  The  graduations  of  the 
circle  consist  of  very  fine  marks  or  lines  all  round  its 
circumference.  The  latter  being  divided  into  three  hun- 
dred and  sixty  degrees,  every  degree  is  marked  by  such 
a  line.  Between  these  it  is  common  to  mark  thirty  inter- 
mediate lines,  which  are  therefore  two  minutes  apart. 
Attached  to  one  or  both  the  stone  piers  which  support 
the  instrument  are  four  microscopes,  so  fixed  that  the 
graduations  on  the  circle  are  seen  through  them.  When 


MERIDIAN    CIRCLE    AND    CLOCK         83 

the  instrument  is  turned  on  its  axis,  all  these  graduations* 
pass  successively  under  each  microscope,  so  that  they 
can  be  seen  by  the  observer  looking  through  the  latter. 
The  position  of  the  star  is  determined  by  measures  with 
the  microscope  on  the  graduation  which  happens  to  be 
under  it  when  the  telescope  is  pointed  at  a  star. 

The  equatorial  telescope  and  the  meridian  circle  are 
the  two  principal  instruments  in  the  astronomical  outfit 
of  an  observatory.  Many  other  instruments  are  more 
or  less  in  use  for  special  purposes,  but  they  are  not  of 
great  interest,  save  to  one  who  is  making  a  special  study 
of  astronomy  and  who  must  therefore  refer  to  books 
specially  written  for  the  professional  student  of  the 
subject. 

The  precision  with  which  a  practised  observer  can 
note  the  time  of  transit  of  a  star  over  the  thread  of  his 
instrument  is  remarkable.  One  method  of  doing  this 
consists  in  listening  to  and  counting  the  beats  of  the 
clock  as  the  star  approaches  and  crosses  the  thread. 
He  watches  the  exact  position  of  the  star  at  the  beat 
before  the  transit,  and  again  at  the  beat  following.  By 
Comparing  in  his  mind  the  opposite  distances  of  the 
star  from  the  thread  at  the  two  clock  beats,  he  estimates 
the  number  of  tenths  of  the  second  at  which  the  transit 
took  place,  and  records  the  time  in  his  notebook. 

This  method  is  now  superseded  in  most  observatories 
by  that  of  registration  on  a  chronograph.  This  instru- 
ment consists  of  a  revolving  cylinder,  covered  with 
paper,  having  a  pen-point  resting  upon  it,  so  that,  as 
the  cylinder  revolves,  the  pen  leaves  a  trace  on  the  paper. 


84,          ASTRONOMICAL    INSTRUMENTS 

The  peri  is  so  connected  with  an  electric  current  passing 
through  the  clock,  and  through  a  key  held  by  the  ob- 
server, that  every  beat  of  the  clock  and  every  pressure 
of  the  key  by  the  observer  makes  a  notch  in  the  trace 
left  by  the  pen.  When  the  observer  sees  that  a  star  has 
reached  the  thread  of  his  instrument  he  presses  the  key, 
and  the  position  of  the  notch  thus  made  in  the  pen-trace 
between  two  notches  made  by  the  clock  gives  the  moment 
at  which  the  key  was  pressed. 

The  astronomer's  clock  must  be  of  the  highest  at- 
tainable perfection,  running  for  a  whole  day  or  more 
without  a  deviation  of  one  tenth  of  a  second.  With  a 
common  house  clock,  the  change  in  the  length  of  the 
pendulum  produced  by  changes  of  temperature  between 
the  day  and  night  would  cause  deviations  of  several 
seconds.  Hence  in  the  astronomical  clock  these  changes 
must  be  neutralised.  This  is  done  by  making  the  pen- 
dulum of  such  a  combination  of  different  materials  that 
the  unequal  expansions  of  the  latter  shall  neutralise  each 
other.  The  most  common  combination  is  that  of  a  steel 
rod  bearing  at  its  lower  end  a  steel  or  glass  jar  of 
quicksilver,  which  serves  as  the  bob  of  the  pendulum. 
Then,  when  the  temperature  rises,  the  upward  expansion 
of  the  quicksilver  compensates  the  downward  expansion 
of  the  steel. 


PART  III 
THE  SUN,  EARTH,  AND  MOON 


I 

AN  INTRODUCTORY  GLANCE  AT  THE  SOLAR  SYSTEM 

WE  have  shown  how  this  comparatively  small  family 
of  bodies,  on  one  of  which  we  dwell,  forms  as  it  were  a 
little  colony  by  itself.  Small  though  it  be  when  com- 
pared with  the  whole  universe  as  a  standard,  it  is  for  us 
the  most  important  part  of  the  universe.  Before  pro- 
ceeding to  a  description  of  its  various  bodies  in  detail 
we  must  take  a  general  view  to  show  of  what  kind  of 
bodies  it  is  formed  and  how  it  is  made  up. 

First  of  all  we  have  the  sun,  the  great  shining  central 
body,  shedding  warmth  and  light  on  all  the  others  and 
keeping  the  whole  system  together  by  virtue  of  its 
powerful  attraction. 

Next  we  have  the  planets,  which  revolve  round  the  sun 
in  their  regular  orbits,  and  of  which  our  earth  is  one. 
The  word  planet  means  wanderer,  a  term  applied  in 
Ancient  times  because  these  bodies,  instead  of  keeping 
their  places  among  the  fixed  stars,  seemed  to  wander 
about  among  them.  The  planets  are  divided  into  two 
quite  distinct  classes,  termed  major  and  minor. 

The  major  planets  are  eight  in  number  and  are,  next 
to  the  sun,  the  largest  bodies  of  the  system.  For  the 
most  part  their  distances  from  the  sun  are  arranged  in  a 
close  approach  to  a  certain  regular  order,  ranging  from 
nearly  forty  millions  of  miles  in  the  case  of  Mercury, 


88         THE    SUN,    EARTH,    AND    MOON 

the  nearest  one,  to  three  thousand  millions  in  the  case  of 
Neptune.  The  latter  is  therefore  seventy  times  as  far 
from  the  sun  as  Mercury.  Still  wider  is  the  range  of 
their  times  of  revolution.  Mercury  performs  its  circuit 
round  the  sun  in  less  than  three  of  our  months — Neptune 
takes  more  than  one  hundred  and  sixty  years  for  his  long 
journey.  It  has  not  yet  made  half  a  revolution  since  its 
discovery  in  1846. 

The  major  planets  are  separated  into  two  groups  of 
four  planets  each,  with  quite  a  broad  gap  between  the 
groups.  The  inner  group  is  composed  of  much  smaller 
planets  than  the  outer  one;  all  four  together  would  not 
make  a  body  one  quarter  the  size  of  the  smallest  of  the 
outer  group. 

In  the  gap  between  the  two  groups  revolve  the  minor 
planets,  or  asteroids  as  they  are  commonly  called.  They 
are  very  small  as  compared  with  the  major  planets.  So 
far  as  we  know  they  are  all  situated  in  a  quite  wide  belt 
ranging  between  a  little  more  than  the  distance  of  the 
earth  out  to  four  times  that  distance.  For  the  most 
part  they  are  about  three  or  four  times  as  far  from  the 
sun  as  the  earth  is.  They  are  also  distinguished  from 
the  major  planets  by  their  indefinite  number;  some  five 
hundred  are  now  known,  and  new  discoveries  are  con- 
tinually being  made  at  such  a  rate  that  no  one  can  set 
any  exact  limit  to  them. 

A  third  class  of  bodies  in  the  solar  system  comprises 
the  satellites,  or  moons.  Several  of  the  major  planets 
have  one  or  more  of  these  small  bodies  revolving  round 
them,  and  therefore  accompanying  them  in  their  revolu- 


GLANCE    AT    THE    SOLAR    SYSTEM      89 

tion  around  the  sun.  The  two  innermost  planets,  Mer- 
cury and  Venus,  have  no  satellites,  so  far  as  we  yet 
know.  In  the  case  of  the  other  planets  their  number 
ranges  from  one  (our  moon)  to  eight,  which  form  the 
retinue  of  the  planet  Saturn.  Each  major  planet,  Mer- 
cury and  Venus  excepted,  is  therefore  the  centre  of  a 
system  bearing  a  certain  resemblance  to  the  solar  system. 
These  systems  are  sometimes  designated  by  names  de- 
rived from  those  of  their  central  bodies.  Thus  we  have 
the  Martian  System,  composed  of  Mars  and  its  satellites ; 
the  Jovian  System,  composed  of  Jupiter  and  its  five 
satellites;  the  Saturnian  System,  comprising  the  planet 
Saturn,  its  rings,  and  satellites. 

A  fourth  class  of  bodies  consists  of  the  comets.  These 
move  round  the  sun  in  very  eccentric  orbits.  We  see 
them  only  on  their  approach  to  the  sun,  which,  in  the 
case  of  most  of  these  bodies,  occurs  only  at  intervals  of 
centuries,  or  even  thousands  of  years.  Even  then  a 
comet  may  fail  to  be  seen  unless  under  favourable 
conditions. 

Besides  the  preceding  bodies  we  have  a  countless  num- 
|>er  of  meteoric  particles  revolving  round  the  sun  in 
regular  orbits.  These  are  probably  related  in  some  way 
to  the  comets.  They  are  completely  invisible  except  as 
they  strike  our  atmosphere,  when  we  see  them  as 
shooting  stars. 

The  following  is  the  arrangement  of  the  planets  in 
the  order  of  their  distance  from  the  sun  and  with  the 
number  of  satellites  of  each: 


90         THE    SUN,    EARTH,    AND    MOON 

/.  Inner  Group  of  Major  Planets: 

Mercury. 

Venus. 

Earth,  with  one  satellite. 

Mars,  with  two  satellites. 

//.  Group  of  Minor  Planets,  or  Asteroids. 

III.  Outer  Group  of  Major  Planets: 

Jupiter,  with  five  satellites. 
Saturn,  with  eight  satellites. 
Uranus,  with  four  satellites. 
Neptune,  with  one  satellite. 

Instead  of  taking  up  these  bodies  in  the  order  of  their 
distance  from  the  sun,  we  shall,  after  describing  the 
latter,  pass  over  Mercury  and  Venus  to  consider  the  earth 
and  moon.  Then  we  shall  return  to  the  other  planets 
and  describe  them  in  order. 


n 

THE  SUN 

IN  a  description  of  the  solar  system  its  great  central 
body  is  naturally  the  first  to  claim  our  attention.  We 
see  that  the  sun  is  a  shining  globe.  The  first  questions 
to  present  themselves  to  us  are  about  the  size  and  dis- 
tance of  this  globe.  It  is  easy  to  state  its  size  when  we 
know  its  distance.  We  know  by  measurement,  the  angle 
subtended  by  the  sun's  diameter.  If  we  draw  two  lines 
making  this  angle  with  each  other,  and  continue  them  in- 
definitely through  the  celestial  spaces,  the  diameter  of 
the  sun  must  be  equal  to  the  distance  apart  of  the  lines 
at  the  distance  of  the  sun.  The  exact  determination  is 
a  very  simple  problem  of  trigonometry.  It  will  suffice 
at  present  to  say  that  the  measure  of  the  apparent 
diameter  of  the  sun,  or  the  angle  which  it  subtends  to  our 
eye,  is  thirty-two  minutes,  making  this  angle  such  that 
the  distance  of  the  sun  is  about  107.5  times  its  diameter 
in  miles.  If,  then,  we  know  the  distance  of  the  sun,  we 
have  only  to  divide  it  by  107.5  to  get  the  sun's  diameter. 

The  various  methods  of  determining  the  distance  of 
the  sun  will  be  described  in  our  chapter  stating  how  dis- 
tances in  the  heavens  are  measured.  The  result  of  all 
the  determinations  is  that  the  distance  is  very  nearly 
ninety-three  million  miles,  perhaps  one  or  two  hundred 
thousand  miles  more.  Taking  the  round  number,  and 


92         THE    SUN,    EARTH,    AND    MOON 

dividing  by  107.5,  we  find  the  diameter  to  be  about  865,- 
000  miles.  This  is  about  one  hundred  and  ten  times  the 
diameter  of  the  earth.  It  follows  that  the  volume  or  bulk 
of  the  sun  is  more  than  one  million  three  hundred  thou- 
sand times  that  of  the  earth. 

The  sun's  importance  to  us  arises  from  its  being  our 
great  source  of  heat  and  light.  Were  these  withdrawn, 
not  only  would  the  world  be  enveloped  in  unending 
night,  but,  in  the  course  of  a  short  time,  in  eternal  frost. 
We  all  know  that  during  a  clear  night  the  surface  of  the 
earth  grows  colder  through  the  radiation  into  space  of 
the  heat  received  from  the  sun  during  the  day.  With- 
out our  daily  supply,  the  loss  of  heat  would  go  on  until 
the  cold  around  us  would  far  exceed  that  which  we  now 
experience  in  the  polar  regions.  Vegetation  would  be 
impossible.  The  oceans  would  freeze  over,  and  all  life 
on  the  earth  would  soon  be  extinct. 

The  surface  of  the  sun,  which  is  all  we  can  see  of  it, 
is  called  the  photosphere.  This  term  is  used  to  distin- 
guish the  visible  surface  from  the  vast  invisible  interior 
of  the  sun.  To  the  naked  eye,  the  photosphere  looks 
entirely  uniform.  But  through  a  telescope  we  see  that 
the  whole  surface  has  a  mottled  appearance,  which  has 
been  aptly  compared  te  that  of  a  plate  of  rice  soup. 
Examination  under  the  best  conditions  shows  that  this 
appearance  is  due  to  minute  and  very  irregular  grains 
which  are  scattered  all  over  the  photosphere. 

When  we  carefully  compare  the  brightness  of  differ- 
ent regions  of  the  photosphere,  we  find  that  the  apparent 
centre  of  the  disk  is  brighter  than  the  edge.  The  differ- 


ROTATION    OF    THE    SUN  93 

ence  can  be  seen  even  without  a  telescope,  if  we  look  at 
the  sun  through  a  dark  glass,  or  when  it  is  setting  in  a 
dense  haze.  The  falling  off  in  the  light  is  especially 
rapid  as  we  approach  the  extreme  edge  of  the  disk,  where 
it  is  little  more  than  half  as  bright  as  at  the  centre. 
There  is  also  a  difference  of  colour,  the  light  of  the  edge 
having  a  lurid  appearance  as  compared  with  that  of 
the  centre. 

All  this  shows  that  the  light  of  the  sun  is  absorbed  by 
an  atmosphere  surrounding  the  sun.  We  readily  see 
that,  the  sun  being  a  globe,  the  light  which  we  receive 
from  the  edge  of  its  disk  leaves  it  obliquely,  while  that 
from  the  centre  leaves  it  perpendicularly.  The  more 
obliquely  the  light  comes  from  the  surface,  the  greater 
the  thickness  of  the  sun's  atmosphere  through  which  it 
must  pass,  and  hence  the  greater  the  portion  lost  by  the 
absorption  of  that  atmosphere.  The  sun's  atmosphere, 
like  our  own,  absorbs  the  green  and  blue  rays  more  than 
the  red.  For  this  reason  the  light  has  a  redder  tint  when 
it  comes  from  near  the  edge  of  the  disk. 

Rotation  of  the  Sun 

Careful  observations  show  that  the  sun,  like  the 
planets,  rotates  on  an  axis  passing  through  its  centre. 
Using  the  same  terms  as  in  the  case  of  the  earth,  we  call 
the  points  in  which  the  axis  intersects  the  surface  the 
poles  of  the  sun,  and  the  circle  around  it  halfway  be- 
tween the  poles  the  sun's  equator.  The  period  of  rota- 
tion is  about  twenty-six  days.  As  the  distance  around 
the  sun  is  more  than  one  hundred  and  ten  times  that 


94         THE    SUN,    EARTH,    AND    MOON 

round  the  earth,  the  speed  of  rotation  must  be  more  than 
four  times  that  of  the  earth's  rotation  to  make  it  com- 
plete the  circuit  in  the  time  that  it  does.  At  the  sun's 
equator  the  speed  is  more  than  a  mile  a  second. 

The  most  curious  feature  of  this  rotation  is  that  it 
is  completed  in  less  time  at  the  equator  than  at  a  distance 
on  each  side  of  the  equator.  Were  the  sun  a  solid  body, 
like  the  earth,  all  its  parts  would  have  to  rotate  at  the 
same  time.  Hence  the  sun  is  not  a  solid  body,  but  must 
be  cither  liquid  or  gaseous,  at  least  at  its  surface. 

The  equator  of  the  sun  is  inclined  six  degrees  to  the 
plane  of  the  earth's  orbit.  Its  direction  is  such  that  in 
our  spring  months  the  north  pole  is  turned  six  degrees 
away  from  us  and  the  central  point  of  the  apparent  disk 
is  about  that  amount  south  of  the  sun's  equator.  In  our 
summer  and  autumn  months  this  is  reversed. 

The  Sun's  Density  and  Gravity 

By  the  mean  density  of  the  sun  we  refer  to  the  average 
specific  gravity  of  the  matter  composing  it,  or  the  ratio 
of  its  weight  to  that  of  an  equal  volume  of  water.  It  is 
known  that  the  density  is  only  about  one  fourth  that  of 
the  earth,  and  about  four  tenths  greater  than  that  of 
water.  Stated  with  more  exactness,  the  figures  are : 

Density  of  sun :  Density  of  earth  =  0.2554. 

Density  of  sun :  Density  of  water  =  1.4115. 

The  mass  or  weight  of  the  sun  is  about  634,000  times 
that  of  the  earth. 

The  force  of  gravity  at  the  sun's  surface  is  27  times 
that  of  the  earth.  If  it  were  possible  for  a  human  being 


SPOTS    ON    THE    SUN  95 

to  be  placed  there,  an  ordinary  man  would  weigh  two 
tons,  and  be  crushed  by  his  own  weight. 

Spots  on  the  Sun 

When  the  sun  is  carefully  examined  with  a  telescope, 
one  or  more  seemingly  dark  spots  will  generally,  though 
not  always,  be  seen  on  its  surface.  These  are,  of  course, 
carried  around  by  the  rotation  of  the  sun,  and  it  is  by 
means  of  them  that  the  time  of  rotation  is  most  easily 
determined.  If  a  spot  appears  at  the  centre  of  the  disk 
it  will,  in  six  days,  be  carried  to  the  western  edge,  and 
there  disappear.  At  the  end  of  about  two  weeks  it  will 
reappear  at  the  eastern  edge  unless  it  has,  in  the  mean- 
time, died  away,  which  is  frequently  the  case. 

The  spots  have  a  wide  range  in  size.  Some  are  very 
minute  points,  barely  visible  in  a  good  telescope,  while  on 
rare  occasions  one  is  large  enough  to  be  seen  with  the 
naked  eye  through  a  dark  glass.  They  frequently  ap- 
pear in  groups,  and  a  group  may  sometimes  be  made  out 
with  the  naked  eye  as  a  minute  patch  when  the  individual 
spots  cannot  be  seen. 

When  the  air  is  steady,  and  a  good-sized  spot  is  care- 
fully examined  with  a  telescope,  it  will  be  seen  to  be  com- 
posed of  a  dark  central  region  or  nucleus,  surrounded 
by  a  shaded  border.  If  all  the  conditions  are  favourable, 
this  border  will  appear  striated,  like  the  edge  of  a 
thatched  roof.  The  appearance  is  represented  in  the  cut, 
which  also  shows  the  mottling  of  the  photosphere. 

The  spots  are  of  the  most  varied  and  irregular  forms, 
frequently  broken  up  in  many  ways.  The  shaded  border, 


96 


THE    SUN,    EARTH,    AND    MOON 


or  the  thatched  lines  which  form  it,  frequently  encroaches 
on  the  nucleus  or  may,  in  places,  extend  quite  across  it. 
A  most  remarkable  law  connected  with  the  spots,  which 
has  been  established  by  nearly  three  centuries  of  observa- 
tion, is  that  their  frequency  varies  in  a  regular  period  of 
eleven  years  and  about  forty  days.  During  a  certain 
year  no  spots  will  be  visible  for  about  half  of  the  time. 


;?IG.  17. — Appearance  of  a  Sun-spot  with  High  Magnifying  Power ,  show- 
ing also  the  Mottling  of  the  Photosphere. 

This  was  the  case  in  1889  and  again  in  1900.  The  year 
following  a  slightly  greater  number  will  show  themselves ; 
and  they  will  increase  year  after  year  for  about  five 
years.  Then  the  frequency  will  begin  to  diminish,  year 
after  year,  until  the  cycle  is  completed,  when  it  will  again 
begin  to  increase.  These  mutations  have  been  traced  back 
to  the  time  of  Galileo,  although  it  was  not  till  about  1825 
that  they  were  found  by  Schwabe  to  take  place  in  a 
regular  period. 


SPOTS    ON    THE    SUN 


97 


Years  of  greatest  and  least  frequency,  past  and  future 
are  as  follows : 


Greatest 

1871 
1882 
1893 
1904 
1916 
1927 


Least 

1878 

1889 

1900 

1911 


1933 


NORTH 


Another  noteworthy  law  connected  with  the  sun's  spots 
is  that  they  are  not  found  all  over  the  sun;  but  only  in 
certain  regions  of  solar  latitude.  They  are  rather  rare 
on  the  sun's  equa- 
tor, but  become 
more  frequent  as  we 
go  north  or  south 
of  the  equator  till 
we  get  to  fifteen  de- 
grees of  latitude, 
north  or  south.  From 
this  region  to  twen- 
ty degrees  the  fre- 
quency is  greatest; 
then  it  falls  off,  so 
that  beyond  thirty 
degrees  a  spot  is 
rarely  seen.  These 
regions  are  shown  in 
the  accompanying  figure,  where  the  shading  is  darker 


FIG.  18. — Frequency  of  Snn-spc     in  Differ 
ent  Latitudes  on  the  Sun. 


98         THE    SUN,    EARTH,    AND    MOON 

the  more  frequent  the  spots.  If  we  made  a  white  globe 
to  represent  the  sun,  and  made  a  black  dot  on  it  for  every 
spot  during  a  number  of  years,  the  dotting  would  make 
the  globe  look  as  represented  in  the  figure. 

The  Faculce 

Collections  of  numerous  small  spots  brighter  than  the 
photosphere  in  general  are  frequently  seen  on  the  sun. 
These  are  often  seen  in  the  neighbourhood  of  a  spot, 
and  occur  most  frequently  in  the  regions  of  greater 
spot  frequency,  but  are  not  entirely  confined  to  those 
regions.  They  are,  however,  rare  near  the  poles  of 
the  sun. 

That  the  spots  and  faculse  proceed  from  some  one 
general  cause  has  been  brought  out  by  the  spectro-helio- 
graph,  an  instrument  devised  by  Professor  George  E. 
Hale  for  taking  photographs  of  the  sun  by  the  light  of 
a  single  ray  of  the  spectrum,  that  emitted  by  calcium,  for 
example.  The  effect  is  the  same  as  if  we  should  look  at 
the  sun  through  a  glass  which  would  allow  the  rays  of 
calcium  vapour  to  pass,  but  would  absorb  all  the  others. 
We  should  then  see  the  calcium  light  of  the  sun  and  no 
other. 

When  the  sun  is  photographed  by  calcium  light  with 
this  instrument,  the  result  is  wonderful.  The  sun-spot 
regions  are  now  seen  to  be  brighter  than  the  others, 
and  faculse  are  found  on  every  part  of  the  sun.  We  thus 
learn  that  eruptions  of  gas,  of  which  calcium  is  the  best 
marked  ingredient,  are  taking  place  all  the  time;  but 
they  arc  more  numerous  in  the  sun-spot  zones  than  else- 


PROMINENCES    AND    CHROMOSPHERE  99 

where.  The  sun-spots  are  therefore  the  effect  of  opera- 
tions going  on  all  the  time,  all  ever  the  sun,  but  giving 
rise  to  a  spot  only  in  the  exceptional  cases  when  they  are 
very  intense. 

It  was  formerly  supposed  that  the  spots  were  openings 
or  depressions  in  the  photosphere,  showing  a  darker 
region  within.  This  view  was  based  on  the  belief  that, 
when  a  spot  was  near  the  edge  of  the  sun's  disk,  the 
shaded  border  next  the  edge  looked  broader  than  the 
other.  But  this  view  is  now  abandoned.  We  cannot  cer- 
tainly say  that  a  spot  is  either  above  or  below  the  photo- 
sphere. We  shall  hereafter  see  that  the  latter  is  not  a 
mere  surface  as  it  seems  to  us,  but  a  shell  or  covering 
many  miles,  perhaps  a  hundred  or  more,  in  thickness. 
The  spots  doubtless  belong  to  this  shell,  being  cooler  por- 
tions of  it,  but  lying  neither  above  nor  below  it. 

The  Prominences  and  Chromosphere 

The  next  remarkable  feature  of  the  sun  to  be  described 
consists  in  the  prominences.  Our  knowledge  of  these  ob- 
jects has  an  interesting  history — which  will  be  mentioned 
-in  describing  eclipses  of  the  sun.  The  spectroscope 
shows  us  that  large  masses  of  incandescent  vapour  burst 
forth  from  every  part  of  the  sun.  They  are  of  such  ex- 
tent that  the  earth,  if  immersed  in  them,  would  be  as  a 
grain  of  sand  in  the  flame  of  a  candle.  They  are  thrown 
up  with  enormous  velocity,  sometimes  hundreds  of  miles 
a  second.  Like  the  faculae,  they  are  more  numerous  in 
the  sun-spot  zones,  but  are  not  confined  to  those  zones. 
The  glare  around  the  sun  caused  by  the  reflection  of  light 


100       THE    SUN,    EARTH,    AND    MOON 

by  the  air  renders  them  entirely  invisible  to  vision,  even 
with  the  telescope,  except  when,  during  total  eclipses  of 
the  sun,  the  glare  is  cut  off  by  the  intervention  of  the 
moon.  They  may  then  be  seen,  even  with  the  naked  eye, 
rising  up  as  if  from  the  black  disk  of  the  moon. 

The  prominences  seem  to  be  of  two  forms,  the  eruptive 
and  the  cloud-like.  The  first  rise  from  the  sun  like  im- 
mense sheets  of  flame ;  the  latter  seem  to  be  at  rest  above 
it,  like  clouds  floating  in  the  air.  But  there  is  no  air 
around  the  sun  for  these  objects  to  float  in,  and  we  can- 
not certainly  say  what  supports  them.  Very  likely,  how- 
ever, it  is  a  repulsive  force  of  the  sun's  rays,  which  will 
be  mentioned  in  a  later  chapter. 

Spectrum  analysis  shows  that  these  prominences  are 
composed  mostly  of  hydrogen  gas,  mixed  with  the  va- 
pours of  calcium  and  magnesium.  It  is  to  the  hydrogen 
that  they  owe  their  red  colour.  Continued  study  of  the 
prominences  shows  them  to  be  connected  with  a  thin  layer 
of  gases  which  surrounds  and  rests  upon  the  photosphere. 
This  layer  is  called  the  chromosphere,  from  its  deep  red 
colour,  similar  to  that  of  the  prominences.  As  in  the  case 
of  the  latter,  most  of  its  light  seems  to  be  that  of  hydro- 
gen; but  it  contains  many  other  substances  in  seemingly 
varying  proportions. 

The  last  appendage  of  the  sun  to  be  considered  is  the 
corona.  This  is  seen  only  during  total  eclipses  as  a  soft 
effulgence  surrounding  the  sun,  and  extending  from  it  in 
long  rays,  sometimes  exceeding  the  diameter  of  the  sun 
in  length.  Its  exact  nature  is  still  in  doubt.  It  will  be 
described  in  the  chapter  on  eclipses. 


HOW    THE    SUN    IS    MADE    UP        101 

How  the  Sun  is  Made  Up 

Let  us  now  recapitulate  what  makes  up  the  sun  as  we 
see  and  know  it. 

We  have  first  the  vast  interior  of  the  globe  which,  of 
course,  we  can  never  see. 

What  we  see  when  we  look  at  the  sun  is  the  shining 
surface  of  this  globe,  the  photosphere.  It  is  not  a  real 
surface,  but  more  likely  a  gaseous  layer  several  hundred 
miles  deep  which  we  cannot  distinguish  from  a  surface. 
This  layer  is  variegated  by  spots,  and  in  or  over  it  rise 
the  f acula?. 

On  the  top  of  the  photosphere  rests  the  layer  of  gases 
called  the  chromosphere,  which  can  be  observed  at  any 
time  with  a  powerful  spectroscope,  but  can  be  seen  by 
direct  vision  only  during  total  eclipses. 

Through  or  from  the  red  chromosphere  are  thrown  up 
the  equally  red  flames  called  the  prominences. 

Surrounding  the  whole  is  the  corona. 

Such  is  the  sun  as  we  see  it.  What  can  we  say  about 
what  it  really  is  ?  First,  is  it  solid,  liquid,  or  gaseous  ? 

That  it  is  not  solid  we  have  already  shown  by  the  law 
of  rotation.  It  cannot  be  a  liquid  like  molten  metal,  be- 
cause it  sends  off  from  its  surface  such  a  flood  of  heat  as 
would  cool  off  and  solidify  molten  metal  in  a  very  short 
time.  For  more  than  thirty  years  it  has  been  understood 
that  the  interior  of  the  sun  must  be  a  mass  of  gas,  com- 
pressed to  the  density  of  a  liquid  by  the  enormous  pres- 
sure of  its  superincumbent  portions.  But  it  was  still  sup- 
posed that  the  photosphere  might  be  in  the  nature  of  a 


1&JTVTHE4  SUN,    EARTH,    AND    MOON 

crust  and  the  whole  sun  like  an  immense  bubble.  This 
view,  however,  seems  no  longer  tenable.  It  does  not  seem 
likely  that  there  is  any  solid  matter  on  the  sun. 

Attempts  have  sometimes  been  made  to  learn  the  tem- 
perature of  the  photosphere.  It  probably  exceeds  any 
that  we  can  produce  on  earth,  even  that  of  the  electric 
furnace,  else  how  could  calcium,  the  metallic  base  of  lime, 
one  of  the  most  refractory  of  substances,  exist  there  in 
a  state  of  vapour?  We  all  know  that  the  air  around  us 
becomes  cooler  and  rarer  as  we  ascend  above  the  surface 
of  the  earth,  owing  to  the  action  of  gravity  and  the  con- 
sequent weight  of  the  atmosphere,  which  gives  rise  to  a 
constantly  increasing  pressure  as  we  descend.  Now, 
gravity  at  the  sun  is  twenty-seven  times  as  powerful  as 
on  the  earth.  Hence,  going  downward,  temperature  and 
pressure  increase  at  a  far  more  rapid  rate  on  the  sun 
than  on  the  earth.  Even  in  the  photosphere  the  tempera- 
ture is  such  that  "the  elements  melt  with  fervent  heat." 
And,  as  we  go  below  the  surface,  the  heat  must  increase 
by  hundreds  of  degrees  for  every  mile  that  we  descend. 
The  result  is  that  in  the  interior  the  gases  of  the  sun  are 
subjected  to  two  opposing  forces  which  grow  more  and 
more  intense.  These  are  the  expansive  force  of  the  heat 
and  the  compressing  force  of  the  gases  above,  produced 
by  the  enormous  force  of  gravity  of  the  sun. 

The  forces  thus  set  in  play  merely  in  the  outer  portions 
of  the  sun's  globe  are  simply  inconceivable.  Perhaps 
the  explosion  of  the  powder  when  a  thirteen-inch  cannon 
is  fired  is  as  striking  an  example  of  the  force  of  ignited 
gases  as  we  are  familiar  with.  Now  suppose  every  foot 


THE    SUN'S    HEAT  103 

of  space  in  a  whole  county  covered  with  such  cannon,  all 
pointed  upward  and  all  being  discharged  at  once.  The 
result  would  compare  with  what  is  going  on  inside  the 
photosphere  about  as  a  boy's  popgun  compares  with  the 
cannon. 

The  Source  of  the  Sun's  Heat 

Perhaps,  from  a  practical  point  of  view,  the  most  com- 
prehensive and  important  problem  of  science  is:  How  is 
the  sun's  heat  kept  up?  Before  the  laws  of  heat  were 
fully  apprehended  this  question  was  not  supposed  to  offer 
any  difficulties.  Even  to  this  day  it  is  supposed  by  those 
not  acquainted  with  the  subject,  that  the  heat  which  we 
receive  from  the  sun  may  arise  in  some  way  from  the  pas- 
sage of  its  rays  through  our  atmosphere,  and  that,  as  a 
matter  of  fact,  the  sun  may  not  radiate  any  actual  heat 
at  all — may  not  be  an  extremely  hot  body.  But,  modern 
science  shows  that  heat  cannot  be  produced  except  by 
the  expenditure  of  some  form  of  energy.  The  energy  of 
the  sun  is  necessarily  limited  in  quantity  and  is  continu- 
ally being  lost  through  radiation. 

It  is  very  easy  to  imagine  the  sun  as  being  something 
like  a  white-hot  cannon  ball,  which  is  cooling  off  by  send- 
ing its  heat  in  all  directions,  as  such  a  ball  does.  We 
know  by  actual  observation  how  much  heat  the  sun  sends 
to  us.  It  may  be  expressed  in  the  following  way: 

Imagine  a  shallow  basin  with  a  flat  bottom,  and  a 
depth  of  one  centimetre,  that  is,  about  four  tenths  of  an 
inch.  Let  the  basin  be  filled  with  water,  the  latter  then 
being  one  centimetre  deep.  Expose  such  a  basin  to  the 


104       THE    SUN,    EARTH,    AND    MOON 

rays  of  the  vertical  sun.  The  heat  which  the  sun  will 
radiate  to  them  will  be  sufficient  to  warm  the  water  about 
three  and  a  half  or  four  degrees  Centigrade,  or  not  very 
far  from  seven  degrees  Fahrenheit,  in  one  minute.  It 
follows  that  if  we  suppose  a  thin  spherical  shell  of  water, 
one  centimetre  thick,  of  the  same  radius  as  the  earth's 
orbit,  and  having  the  sun  in  its  centre,  that  shell  of  water 
will  be  heated  with  the  rapidity  just  mentioned.  The 
heat  which  it  receives  will  be  the  total  amount  radiated 
by  the  sun.  We  can  thus  define  how  much  heat  the  sun 
loses  every  minute,  day  and  year. 

A  very  simple  calculation  will  show  that  if  the  sun 
were  of  the  nature  of  a  white-hot  ball  it  would  cool  off  so 
rapidly  that  its  heat  could  not  last  more  than  a  few  cen- 
turies. But  it  has  in  all  probability  lasted  millions  of 
years.  Whence,  then,  comes  the  supply?  The  answer 
of  modern  science  to  this  question  is  that  the  heat  radi- 
ated from  the  sun  is  supplied  by  the  contraction  of  size 
as  heat  is  lost.  We  all  know  that  in  many  cases  when  mo- 
tion is  destroyed  heat  is  produced.  When  a  cannon  shot 
is  fired  at  the  armour  plate  of  a  ship  of  war,  the  mere 
stroke  of  the  shot  makes  both  plate  and  shot  hot.  The 
blacksmith  can  make  iron  hot  by  hammering  it. 

These  facts  have  been  generalized  into  the  statement 
that  whenever  a  body  falls  and  is  stopped  in  its  fall  by 
friction,  or  by  a  stroke  of  any  sort,  heat  is  produced. 
From  the  law  governing  the  case,  we  know  that  the  water 
of  Niagara,  after  it  strikes  the  bottom  of  the  falls,  must 
be  about  one  quarter  of  a  degree  warmer  than  it  was 
during  the  fall.  We  also  know  that  a  hot  body  contracts 


THE    SUN'S    HEAT  105 

in  volume  when  cooled.  The  contraction  of  a  gaseous 
body,  such  as  we  believe  the  sun  to  be,  is  greater  than 
that  of  a  solid  or  liquid.  The  heat  of  the  sun  is  radiated 
from  streams  of  matter  constantly  rising  from  the  in- 
terior, which  radiate  their  heat  when  they  reach  the  sur- 
face. Being  cooled  they  fall  back  again,  and  the  heat 
caused  by  this  fall  is  what  keeps  the  sun  hot. 

It  may  seem  almost  impossible  that  heat  sufficient  to 
last  for  millions  of  years  could  be  generated  in  this  way ; 
but  the  known  force  of  gravity  at  the  surface  of  the  sun 
enables  us  to  make  exact  computations  on  the  subject. 
It  is  thus  found  that  in  order  to  keep  up  the  supply  of 
heat  it  is  only  necessary  that  the  diameter  of  the  sun 
should  contract  about  a  mile  in  twenty-five  years — or 
four  miles  in  a  century.  This  amount  would  not  be  per- 
ceptible until  after  thousands  of  years.  Yet  the  process 
of  contraction  must  come  to*  an  end  some  time.  There- 
fore, if  this  view  is  correct,  the  life  of  the  sun  must  have 
a  limit.  What  its  limit  may  be  we  cannot  say  with  exact- 
ness, we  only  know  that  it  is  several  millions  of  years,  but 
not  many  millions. 

^  The  same  theory  implies  that  the  sun  was  larger  in 
former  times  than  it  is  now,  and  must  have  been  larger 
and  larger  every  year  that  we  go  back  into  its  history. 
There  was  a  time  when  it  must  have  been  as  large  as  the 
whole  solar  system.  In  this  case  it  could  have  been 
nothing  but  a  nebula.  We  thus  have  the  theory  that  the 
sun  and  solar  system  have  resulted  from  the  contraction 
of  a  nebula — through  millions  of  years.  This  view  is 
familiarly  known  as  the  nebular  hypothesis. 


106       THE    SUN,    EARTH,    AND    MOON 

The  question  whether  the  nebular  hypothesis  is  to  be 
accepted  as  a  proved  result  of  science  is  one  on  which 
opinions  differ.  There  are  many  facts  which  support  it 
— such  as  the  interior  heat  of  the  earth  and  the  revolu- 
tion and  rotation  of  the  planets  all  in  the  same  direction. 
But  cautious  and  conservative  minds  will  want  some  fur- 
ther proof  of  the  theory  before  they  regard  it  as  abso- 
lutely established.  Even  if  we  accept  it,  we  still  have 
open  the  question:  How  did  the  nebula  itself  originate, 
and  how  did  it  begin  to  contract  ?  This  brings  us  to  the 
boundary  where  science  can  propound  a  question  but 
cannot  answer  it. 


m 

THE  EARTH 

THE  globe  on  which  we  live,  being  one  of  the  planets, 
would  be  entitled  to  a  place  among  the  heavenly  bodies 
even  if  it  had  no  other  claims  on  our  attention.  Insig- 
nificant though  it  is  in  size  when  compared  with  the  great 
bodies  of  the  universe,  or  even  with  the  four  giant  planets 
of  our  system,  it  is  the  largest  of  the  group  to  which  it 
belongs.  Of  the  rank  which  it  might  claim  as  the  abode 
of  man  we  need  not  speak. 

What  is  the  earth?  We  may  describe  it  in  the  most 
comprehensive  way  as  a  globe  of  matter  nearly  eight 
thousand  miles  in  diameter,  bound  together  by  the  mu- 
tual gravitation  of  its  parts.  We  all  know  that  it  is  not 
exactly  spherical,  but  bulges  out  very  slightly  at  the 
equator.  The  problem  of  determining  its  exact  shape 
and  size  is  an  extremely  difficult  one,  and  we  cannot  say 
that  an  entirely  satisfactory  result  is  yet  reached.  The 
difficulty  is  obvious  enough.  There  is  no  way  of  measur- 
ing distances  across  the  great  oceans.  The  measurements 
arc  necessarily  limited  to  such  islands  as  are  visible  from 
the  coasts  of  the  continents  or  from  each  other.  Of 
course,  the  measures  cannot  be  extended  to  either  pole. 
The  size  and  shape  must  therefore  be  inferred  from  the 
measures  across  or  along  the  continents.  Owing  to  the 
importance  of  such  work,  the  leading  nations  have  from 


108       THE    SUN,    EARTH,    AND    MOON 

time  to  time  entered  into  it.  Quite  recently  our  Coast 
and  Geodetic  Survey  has  completed  the  measurement  of  a 
line  of  triangles  extending  from  the  Atlantic  to  the 
Pacific  Oceans.  North  and  south  measurements  both  on 
the  Atlantic  and  Pacific  coasts  have  been  executed  or  are 
in  progress.  The  English  have  from  time  to  time  made 
measures  of  the  same  sort  in  Africa,  and  the  Russians 
and  Germans  on  their  respective  territories.  Nearly  all 
these  measures  are  now  being  combined  in  a  work  carried 
on  by  the  International  Geodetic  Association,  of  which 
the  geodetic  authorities  of  the  principal  countries  are 
members. 

The  latest  conclusions  on  the  subject  may  be  summed 
up  thus.  We  remark  in  the  first  place  that  by  the 
figure  of  the  earth  geodetists  do  not  mean  the  figure 
of  the  continents,  but  of  the  ocean  level  as  it  would 
be  if  canals  admitting  the  water  of  the  oceans  were 
dug  through  the  continents.  The  earth  thus  defined  is 
approximately  an  ellipsoid,  of  which  the  smaller  diameter 
is  that  through  the  poles,  and  which  has  about  the 
following  dimensions: 

Polar  diameter,  7,899.6  miles,  or  12,713.0  kilometres. 

Equatorial     "    7,926.6  miles,  or  12,756.5  kilometres. 

It  will  be  seen  that  the  equatorial  diameter  is  twenty- 
seven  miles  or  forty-three  kilometres  greater  than  the 
polar. 

The  Earth's  Interior 

What  we  know  of  the  earth  by  direct  observation  is 
confined  almost  entirely  to  its  surface.  The  greatest 
depth  to  which  man  has  ever  been  able  to  penetrate  com- 


THE    EARTH'S    INTERIOR  109 

pares  with  the  size  of  the  globe  only  as  the  skin  of  an 
apple  does  to  the  body  of  the  fruit  itself. 

I  shall  first  invite  the  reader's  attention  to  some  facts 
about  weight,  pressure,  and  gravity  in  the  earth.  Let 
us  consider  a  cubic  foot  of  soil  forming  part  of  the  outer 
surface  of  the  earth.  This  upper  cubic  foot  presses  upon 
its  bottom  with  its  own  weight,  perhaps  one  hundred  and 
fifty  pounds.  The  cubic  foot  below  it  weighs  an  equal 
amount,  and  therefore  presses  on  its  bottom  with  a  force 
equal  to  its  own  weight  with  the  weight  of  the  other  foot 
added  to  it.  This  continual  increase  of  pressure  goes  on 
as  we  descend.  Every  square  foot  in  the  earth's  interior 
sustains  a  pressure  equal  to  the  weight  of  a  column  of 
the  earth  a  foot  square  extending  to  the  surface.  Not 
many  yards  below  the  surface  this  pressure  will  be  meas- 
ured in  tons ;  at  the  depth  of  a  mile  it  may  be  thirty  or 
forty  tons ;  at  the  depth  of  one  hundred  miles,  thou- 
sands of  tons ;  continually  increasing  to  the  centre.  Un- 
der this  enormous  pressure  the  matter  composing  the 
inner  portion  of  the  earth  is  compressed  to  the  density  of 
a  metal.  By  a  process  which  we  will  hereafter  describe, 

e  mean  density  of  the  earth  is  known  to  be  five  and  one 
half  times  that  of  water,  while  the  superficial  density  is 
only  two  or  three  times  that  of  water. 

One  of  the  most  remarkable  facts  about  the  earth  is 
that  the  temperature  continually  increases  as  we  pene- 
trate below  the  surface  in  deep  mines.  The  rate  of  in- 
crease is  different  in  different  latitudes  and  regions.  The 
general  average  is  one  degree  Fahrenheit  in  fifty  or  sixty 
feet. 


110       THE    SUN,    EARTH,    AND    MOON 

The  first  question  to  suggest  itself  is,  how  far  toward 
the  earth's  centre  does  this  increase  of  temperature  ex- 
tend? The  most  that  we  can  say  is  that  it  cannot  be 
merely  superficial,  because,  in  that  case,  the  exterior  por- 
tions would  have  cooled  off  long  ago,  so  that  we  should 
have  no  considerable  increase  of  heat  as  we  went  down. 
The  fact  that  the  heat  has  been  kept  up  during  the  whole 
of  the  earth's  existence  shows  that  it  must  still  be  very 
intense  toward  the  centre,  and  that  the  rate  of  increase 
near  the  surface  must  go  on  for  many  miles  into  the 
interior. 

At  this  rate  the  material  of  the  earth  would  be  red  hot 
at  a  depth  of  ten  or  fifteen  miles,  while  at  one  or  two 
hundred  miles  the  heat  would  be  sufficient  to  melt  all  the 
substances  which  form  the  earth's  crust.  This  fact  sug- 
gested to  geologists  the  idea  that  our  globe  is  really  a 
molten  mass,  like  a  mass  of  melted  iron,  covered  by  a  cool 
crust  a  few  miles  thick,  on  which  we  dwell.  The  exist- 
ence of  volcanoes  and  the  occurrence  of  earthquakes 
gave  additional  weight  to  this  view,  as  did  also  othei 
geological  evidence,  showing  changes  in  the  earth's 
surface  which  appeared  to  be  the  result  of  a  liquic 
interior. 

But  in  recent  years  the  astronomer  and  physicist  have 
collected  evidence,  which  is  as  conclusive  as  such  evidence 
can  be,  that  the  earth  is  solid  from  centre  to  surface,  anc 
even  more  rigid  than  a  similar  mass  of  steel.  The  sub- 
ject was  first  developed  most  fully  by  Lord  Kelvin,  who 
showed  that,  if  the  earth  were  a  fluid,  surrounded  by  a 
crust,  the  action  of  the  moon  would  not  cause  tides  in  the 


EARTH'S  GRAVITY  AND  DENSITY     111 

ocean,  but  would  merely  tend  to  stretch  out  the  entire 
earth  in  the  direction  of  the  moon,  leaving  the  relative 
positions  of  the  crust  and  the  water  unchanged. 

Equally  conclusive  is  the  curious  phenomenon  which 
we  shall  describe  presently  of  the  variation  of  latitudes 
on  the  earth's  surface.  Not  only  a  globe  of  which  the 
interior  is  soft,  but  even  a  globe  no  more  rigid  than  steel 
could  not  rotate  as  the  earth  does. 

How,  then,  are  we  to  reconcile  the  enormous  tempera- 
ture and  the  solidity?  There  seems  to  be  only  one  solu- 
tion possible.  The  matter  of  the  interior  of  the  earth 
is  kept  solid  by  the  enormous  pressure.  It  is  found  ex- 
perimentally that  when  masses  of  matter  like  the  rocks 
of  the  earth  are  raised  to  the  melting  point,  and  then 
subjected  to  heavy  pressure,  the  effect  of  the  pressure  is 
to  make  them  solid  again.  Thus,  as  we  increase  the  tem- 
perature we  have  only  to  increase  the  pressure  also  to 
keep  the  material  of  the  earth  solid.  And  thus  it  is  that, 
as  we  descend  into  the  earth,  the  increase  of  pressure 
more  than  keeps  pace  with  the  rise  of  temperature,  and 
thus  keeps  the  whole  mass  solid. 

Gravity  and  Density  of  the  Earth 

Another  interesting  question  connected  with  the  earth 
is  that  of  its  density,  or  specific  gravity.  We  all  know 
that  a  lump  of  lead  is  heavier  than  an  equal  lump  of  iron, 
and  the  latter  heavier  than  an  equal  lump  of  wood.  Is 
there  any  way  of  determining  what  a  cubic  foot  of  earth 
would  weigh  if  taken  out  from  a  great  depth  of  its  vast 
interior?  If  there  is,  then  we  can  determine  what  the 


THE    SUN,   EARTH,   AND    MOON 

actual  weight  of  the  whole  earth  is.  The  solution  de- 
pends on  the  gravitation  of  matter. 

Every  child  is  familiar  with  gravitation  from  the  time 
it  begins  to  walk,  but  the  profoundest  philosopher  knows 
nothing  of  its  cause,  and  science  has  not  discovered  any- 
thing respecting  it  except  a  few  general  facts.  The 
widest  and  most  general  of  these  facts,  which  may  be  said 
to  include  the  whole  subject,  is  Sir  Isaac  Newton's  theory 
of  gravitation.  According  to  this  theory,  the  mysterious 
force  by  which  all  bodies  on  the  surface  of  the  earth  tend 
to  fall  toward  its  centre  does  not  reside  merely  in  the 
centre  of  the  earth,  but  is  due  to  an  attraction  exerted 
by  every  particle  of  matter  composing  our  globe. 
Whether  this  was  the  case  was  at  first  an  open  question. 
Even  so  great  a  philosopher  and  physicist  as  Huyghens 
believed  that  the  power  resided  in  the  earth's  centre,  and 
not  in  every  particle,  as  Newton  supposed.  But  the  lat- 
ter extended  his  theory  yet  farther  by  showing  that  every 
particle  of  matter  in  the  universe,  so  far  as  we  have  yet 
ascertained,  attracts  every  other  particle  with  a  force 
that  diminishes  as  the  square  of  the  distance  increases. 
This  means  that  at  twice  the  distance  the  attraction  will 
be  divided  by  four ;  at  three  times  by  nine ;  at  four  times 
by  sixteen,  and  so  on. 

Granting  this,  it  follows  that  all  objects  around  us 
have  their  own  gravitating  power,  and  the  question 
arises :  Can  we  show  this  power  by  experiment,  and  meas- 
ure its  amount  ?  The  mathematical  theory  shows  that 
globes  should  attract  small  bodies  at  their  surfaces  with 
a  force  proportioned  to  their  diameter.  A  globe  two 


ATTRACTION    OF    THE    EARTH        113 

feet  in  diameter,  of  the  same  specific  gravity  as  the  earth, 
should  attract  with  a  force  one  twenty-millionth  of  the 
earth's  gravity. 

In  recent  times  several  physicists  have  succeeded  in 
measuring  the  attraction  of  globes  of  lead  having  a 
diameter  of  a  foot,  more  or  less.  This  measurement  is 
tht  most  delicate  and  difficult  that  has  ever  been  made, 
and  the  accuracy  which  seems  to  have  been  reached  would 
have  been  incredible  a  few  years  ago.  The  apparatus 
used  is,  in  its  principle,  of  the  simplest  kind.  A  very 
light  horizontal  rod  is  suspended  at  its  centre  by  a  thread 
of  the  finest  and  most  flexible  material  that  can  be  ob- 
tained. This  rod  is  balanced  by  having  a  small  ball  at- 
tached to  each  end.  What  is  measured  is  the  attraction 
of  the  globes  of  lead  upon  these  two  balls.  The  former 
are  placed  in  such  a  position  as  to  unite  their  attraction 
in  giving  the  rod  a  slight  twisting  motion  in  the  horizon- 
tal plane.  To  appreciate  the  difficulties  of  the  case,  we 
must  call  to  mind  that  the  attraction  may  not  amount  to 
the  ten-millionth  part  of  the  weight  of  the  little  balls. 
It  would  be  difficult  to  find  any  object  so  light  that  its 
weight  would  not  exceed  this  force.  To  compare  the 
weight  of  a  fly  with  it  would  be  like  comparing  the 
weight  of  an  ox  with  that  of  a  dose  of  medicine.  Not 
only  the  weight  of  a  mosquito  but  even  of  its  finest  limb 
might  exceed  the  quantity  to  be  measured.  If  a  mosquito 
were  placed  under  a  microscope  an  expert  operator  could 
cut  off  from  one  antenna  a  piece  small  enough  to  express 
the  force  measured. 

Yet  the  determination  of  this  force  has  been  made  with 


THE    SUN,    EARTH,    AND    MOON 

such  precision  that  the  results  of  the  two  latest  investiga- 
tors do  not  differ  by  a  thousandth  part.  These  were 
Professor  Boys,  F.R.S.,  of  Oxford,  England,  and 
Dr.  Karl  Braun,  S.J.,  of  Marienschein,  in  Bohemia. 
They  worked  independently  at  the  problem,  meeting 
and  overcoming  innumerable  difficulties  one  after  another, 
getting  greater  and  greater  delicacy  and  precision  in 
their  apparatus,  and  finally  published  their  results  al- 
most at  the  same  time,  the  one  in  England,  the  other  in 
Austria.  The  outcome  of  their  experiments  is  that  the 
mean  density  of  the  earth  is  slightly  more  than  five  and 
a  half  times  that  of  water.  This  is  a  little  less  than  the 
density  of  iron,  but  much  more  than  that  of  any  or- 
dinary stone.  As  the  mean  density  of  the  materials 
which  compose  the  earth's  crust  is  scarcely  more  than 
one  half  of  this  amount,  it  follows  that  near  the  centre 
the  matter  composing  the  earth  must  be  compressed  to 
a  density  not  only  far  exceeding  that  of  iron,  but  prob- 
ably that  of  lead. 

The  attraction  of  mountains  has  been  known  for  more 
than  a  hundred  years.  It  was  first  demonstrated  by 
Maskelyne  about  1775  in  the  case  of  Mount  Schehallion, 
in  Scotland.  In  all  mountain  regions  where  very  accu- 
rate surveys  are  made  the  attraction  of  mountains  upon 
the  plumb  line  is  very  evident. 

Variations  of  Latitude 

We  know  that  the  earth  rotates  on  an  axis  passing 
through  the  centre  and  intersecting  the  earth's  surface  at 
either  pole.  If  we  imagine  ourselves  standing  exactly 


VARIATIONS    OF    LATITUDE  115 

on  a  pole  of  the  earth,  with  a  flagstaff  fastened  in  the 
ground,  we  should  be  carried  round  the  flagstaff  by  the 
earth's  rotation  once  in  twenty-four  hours.  We  should 
become  aware  of  the  motion  by  seeing  the  sun  and  stars 
apparently  moving  in  the  opposite  direction  in  horizontal 
circles  by  virtue  of  the  diurnal  motion.  Now,  the  great 
discovery  of  the  variation  of  latitude  is  this :  The  point 
in  which  the  axis  of  rotation  intersects  the  surface  is  not 
fixed,  but  moves  around  in  a  somewhat  variable  and  ir- 
regular curve,  contained  within  a  circle  nearly  sixty  feet 
in  diameter.  That  is  to  say,  if  standing  at  the  north 
pole  we  should  observe  its  position  day  by  day,  we  should 
find  it  moving  one,  two,  or  three  inches  every  day,  de- 
scribing in  the  course  of  time  a  curve  around  one  central 
point,  from  which  it  would  sometimes  be  farther  away 
and  sometimes  nearer.  It  would  make  a  complete  revolu- 
tion in  this  irregular  way  in  about  fourteen  months. 

Since  we  have  never  been  at  the  pole,  the  question 
may  arise :  How  is  this  known  ?  The  answer  is  that  by 
astronomical  observations  we  can,  on  any  night,  deter- 
mine the  exact  angle  between  the  plumb  line  at  the  place 
where  we  stand  and  the  axis  on  which  the  earth  is  rota- 
tifig  on  that  particular  day.  Four  or  five  stations  for 
making  these  observations  were  established  around  the 
earth  in  1900  by  the  International  Geodetic  Association. 
One  of  these  stations  is  near  Gaithersburg,  Md.,  another 
is  on  the  Pacific  coast,  a  third  is  in  Japan,  and  a  fourth 
in  Italy.  Before  these  were  established  observations 
having  the  same  object  were  made  in  various  parts  of 
Europe  and  America.  The  two  most  important  stations 


116       THE    SUN,    EARTH,    AND    MOON 

in  the  latter  region  were  those  of  Professor  Rees  of  Co- 
lumbia University,  New  York,  and  of  Professor  Doolittle, 
first  at  Lehigh,  and  later  at  the  Flower  Observatory, 
near  Philadelphia. 

The  variation  which  we  have  described  was  originally 
demonstrated  by  S.  C.  Chandler,  of  Cambridge,  in  1890 
by  means  of  a  great  mass  of  astronomical  observations 
not  made  for  this  special  purpose.  Since  then  investi- 
gation has  been  going  on  with  the  view  of  determining 
the  exact  curve  described.  What  has  been  shown  thus 
far  is  that  the  variation  is  much  wider  some  years  than 
others,  being  quite  considerable  in  1891,  and  very  small 
in  1894.  It  appears  that  in  the  course  of  seven  years 
there  will  be  one  in  which  the  pole  describes  the  greater 
part  of  a  comparatively  wide  circle,  while  three  or  four 
years  later  it  will  for  several  months  scarcely  move  from 
its  central  position. 

If  the  earth  were  composed  of  a  fluid,  or  even  of  a 
substance  which  would  bend  no  more  than  the  hardest 
steel,  such  a  motion  of  the  axis  as  this  would  be  impossi- 
ble. Our  globe  must  therefore,  in  the  general  average, 
be  more  rigid  than  steel. 

The  Atmosphere 

The  atmosphere  is  astronomically,  as  well  as  physic- 
ally, a  most  important  appendage  of  the  earth.  Neces- 
sary though  it  is  to  our  life  it  constitutes  one  of  the 
greatest  obstructions  with  which  the  astronomer  has  to 
deal.  It  absorbs  more  or  less  of  all  the  light  that  passes 
through  it,  and  thus  slightly  changes  the  colour  of  the 


THE    ATMOSPHERE  117 

heavenly  objects  as  we  see  them,  and  renders  them  some- 
what dimmer,  even  in  the  clearest  sky.  It  also  refracts 
the  light  passing  through  it,  causing  it  to  describe  a 
slightly  curved  line,  concave  toward  the  earth,  instead  of 
passing  straight  to  the  astronomer's  eye.  The  result  of 
this  is  that  the  stars  appear  slightly  higher  above  the 
horizon  than  they  actually  are.  The  light  coming  directly 
down  from  a  star  in  the  zenith  suffers  no  refraction.  The 
latter  increases  as  the  star  is  farther  from  the  zenith, 
but  even  forty-five  degrees  away  it  is  only  one  minute 
of  arc,  about  the  smallest  amount  that  the  unaided  eye 
can  plainly  perceive;  yet  this  is  a  very  important  quan- 
tity to  the  astronomer.  The  nearer  the  object  is  to  the 
horizon  the  greater  the  rate  at  which  the  refraction  in- 
creases; twenty-eight  degrees  above  the  horizon  it  is 
about  twice  as  great  as  at  forty -five  degrees ;  at  the  hori- 
zon it  is  more  than  one  half  a  degree,  that  is  more  than 
the  whole  diameter  of  the  sun  or  moon.  The  result  is  that 
when  we  see  the  sun  just  about  to  touch  the  horizon  at 
sunset  or  sunrise  its  whole  body  is  in  reality  below  the 
horizon.  We  see  it  only  in  consequence  of  the  refraction 
of  its  light.  Another  result  of  the  rapid  increase  near 
the  horizon  is  that,  in  this  position,  the  sun  looks  decid- 
edly flattened  to  the  eye,  its  vertical  diameter  being 
shorter  than  the  horizontal  one.  Anyone  may  notice 
this  who  has  an  opportunity  to  look  at  the  sun  as  it  is 
setting  in  the  ocean.  It  arises  from  the  fact  that  the 
lower  edge  of  the  sun  is  refracted  more  than  the  upper 
edge. 

When  the  sun  sets  in  the  ocean  in  the  clear  air  of  the 


118       THE    SUN,    EARTH,   AND    MOON 

tropics  a  beautiful  effect  may  be  noticed,  which  can 
rarely  or  never  be  seen  in  the  thicker  air  of  our  latitudes. 
It  arises  from  the  unequal  refraction  of  the  rays  of  light 
by  the  atmosphere.  Like  a  prism  of  glass  the  atmos- 
phere refracts  the  red  rays  the  least  and  the  successive 
spectral  colours,  yellow,  green,  blue,  and  violet,  more 
and  more.  The  result  is  that,  as  the  edge  of  the  sun  is 
disappearing  in  the  ocean,  these  successive  rays  are  lost 
sight  of  in  the  same  order.  Two  or  three  seconds  before 
the  sun  has  disappeared,  the  little  spark  of  its  limb 
which  still  remains  visible  is  seen  to  change  colour  and 
rapidly  grow  paler.  This  tint  changes  to  green  and 
blue,  and  finally  the  last  glimpse  which  we  see  is  that  of 
a  disappearing  flash  of  blue  or  violet  light. 


rv 

THE  MOON 

ABOUT  one  hundred  years  ago  there  was  an  unpopular 
professor  in  the  Government  Polytechnique  School  of 
Paris,  still  the  great  school  of  mathematics  for  the 
French  public  service,  who  loved  to  get  his  students  into 
difficulties.  One  morning  he  addressed  one  of  them  the 
question : 

"Monsieur,  have  you  ever  seen  the  moon?" 

"No,  sir,"  replied  the  student,  suspecting  a  trap. 

The  professor  was  nonplussed.  "Gentlemen,"  said  he, 

"see  Mr. ,  who  professes  never  to  have  seen  the 

moon !" 

The  class  all  smiled. 

"I  admit  that  I  have  heard  it  spoken  of,"  said  the 
student,  "but  I  have  never  seen  it." 

I  take  it  for  granted  that  the  reader  has  been  more 
observant  than  the  French  student  professed  to  be,  and 
that  he  has  not  only  seen  the  moon,  but  knows  the  phases 
through  which  it  goes  and  is  familiar  with  the  fact  that 
it  describes  a  monthly  course  around  the  earth.  I  also 
suppose  that  he  knows  the  moon  to  be  a  globe,  although, 
to  the  naked  eye,  it  seems  like  a  flat  disk.  The  globular 
form  is,  however,  very  evident  when  we  look  at  it  with  a 
small  telescope. 

Various  methods  and  systems  of  measurement  all  agree 


120       THE    SUN,    EARTH,    AND    MOON 

in  placing  the  moon  at  an  average  distance  of  a  little 
less  than  two  hundred  and  forty  thousand  miles.  This 
distance  is  obtained  by  direct  measure  of  the  paral- 
lax, as  will  be  explained  hereafter,  and  also  by  calcula- 
ting how  far  off  the  moon  must  be  in  order  that,  being 
projected  into  space,  it  may  describe  an  orbit  around  the 
earth  in  the  time  that  it  actually  does  perform  its 
round.  The  orbit  is  elliptic,  so  that  the  actual  distance 
varies.  Sometimes  it  is  ten  or  fifteen  thousand  miles  less, 
at  other  times  as  much  more,  than  the  average. 

The  diameter  of  the  moon's  globe  is  a  little  more  than 
one  fourth  that  of  the  earth;  more  exactly,  it  is  two 
thousand  one  hundred  and  sixty  miles.  The  most  careful 
measures  show  no  deviation  from  the  globular  form 
except  that  the  surface  is  very  irregular. 

Revolution  and  Phases  of  the  Moon 

The  moon  accompanies  the  earth  in  its  revolution 
round  the  sun.  To  some  the  combination  of  the  two 
motions  seems  a  little  complex ;  but  it  need  not  offer  any 
real  difficulty.  Imagine  a  chair  standing  in  the  centre 
of  a  railway  car  in  rapid  motion,  while  a  person  is  walk- 
ing around  it  at  a  distance  of  three  feet.  He  can  go 
round  and  round  without  varying  his  distance  from  the 
chair  and  without  any  difficulty  arising  from  the  motion 
of  the  car.  Thus  the  earth  moves  forward  in  its  orbit, 
and  the  moon  continually  revolves  around  it  without 
greatly  varying  its  distance  from  us. 

The  actual  time  of  the  moon's  revolution  around  the 
earth  is  twenty -seven  days  eight  hours;  but  the  time 


MOON'S  REVOLUTION  AND  PHASES    121 

from  one  new  moon  to  another  is  twenty-nine  days  thir- 
teen hours.  The  difference  arises  from  the  earth's  mo- 
tion around  the  sun ;  or,  which  amounts  to  the  same  thing, 
t}\p  apparent  motion  of  the  sun  along  the  ecliptic.  To 


FIG.  19. — Revolution  of  the  Moon  Round  the  Earth. 

show  this,  let  AC  be  a  small  arc  of  the  earth's  orbit 
around  the  sun.  Suppose  that  at  a  certain  time  the  earth 
is  at  the  point  E,  and  the  moon  at  the  point  M,  between 
the  earth  and  the  sun.  At  the  end  of  twenty-seven  days 
eight  hours  the  earth  will  have  moved  from  E  to  F. 


122       THE    SUN,    EARTH,    AND    MOON 

While  the  earth  is  making  this  motion  the  moon  will  have 
moved  around  the  orbit  in  the  direction  of  the  arrows, 
so  as  to  have  reached  the  point  N.  At  the  moment  when 
the  lines  EM  and  FN  are  parallel  to  each  other,  the  moon 
will  have  completed  her  actual  revolution,  and  will  seem 
to  be  in  the  same  place  among  the  stars  as  before.  But 
the  sun  is  now  in  the  direction  FS.  The  moon  therefore 
has  to  continue  its  motion  before  it  catches  up  to  the  sun. 
This  requires  a  little  more  than  two  days,  and  makes 
the  whole  time  between  two  new  moons  twenty-nine  and 
a  half  days. 

The  varying  phases  of  the  moon  depend  upon  its 
position  with  respect  to  the  sun.  Being  an  opaque  globe, 
without  light  of  its  own,  we  see  it  only  as  the  light  of 
the  sun  illuminates  it.  When  it  is  between  us  and  the 
sun  its  dark  hemisphere  is  turned  toward  us,  and  it  is 
entirely  invisible.  The  time  of  this  position  in  the 
almanacs  is  called  "new  moon,"  but  we  cannot  commonly 
see  the  moon  for  nearly  two  days  after  this  time,  because 
it  is  lost  in  the  bright  twilight  of  evening.  On  the  second 
and  third  day,  however,  we  see  a  small  portion  of  the 
illuminated  globe,  having  the  familiar  form  of  a  thin 
crescent.  This  crescent  we  commonly  call  the  new  moon, 
although  the  time  given  in  the  almanac  is  several  days 
earlier. 

In  this  position,  and  for  several  days  longer,  we  may, 
if  the  sky  is  clear,  see  the  entire  face  of  the  moon,  the 
dark  parts  shining  with  a  faint  gray  light.  This  light 
is  that  which  is  reflected  from  the  earth  to  the  moon. 
An  inhabitant  of  the  moon,  if  there  were  such,  would 


SURFACE    OF    THE    MOON 

then  see  the  earth  in  the  sky  like  a  full  moon,  looking 
much  larger  than  the  moon  looks  to  us.  As  the  moon 
advances  in  its  orbit  day  after  day,  this  light  diminishes, 
and  about  the  time  of  first  quarter  disappears  from  our 
sight  owing  to  the  brightness  of  the  illuminated  portion 
of  the  moon. 

Seven  or  eight  days  after  the  almanac  time  of  new 
moon,  the  moon  reaches  its  first  quarter.  We  then  see 
half  of  the  illuminated  disk.  During  the  week  following, 
the  moon  has  the  form  called  gibbous.  At  the  end  of  the 
second  week  the  moon  is  opposite  the  sun,  and  we  see  its 
entire  hemisphere  like  a  round  disk.  This  we  call  full 
moon.  During  the  remainder  of  its  course  the  phases 
recur  in  reverse  order,  as  we  all  know. 

We  might  regard  all  these  recurrences  as  too  well 
known  to  need  description,  yet,  in  the  Ancient  Mariner, 
a  star  is  described  as  seen  between  the  two  horns  of  the 
moon  as  though  there  were  no  dark  body  there  to  inter- 
cept our  view  of  the  star.  Probably  more  than  one  poet 
has  described  the  new  moon-  as  seen  in  the  eastern  sky, 
or  the  evening  full  moon  as  seen  in  the  west. 

The  Surface  of  the  Moon 

We  can  see  with  the  naked  eye  that  the  moon's  surface 
is  variegated  by  bright  and  dark  regions.  The  latter 
are  sometimes  conceived  to  have  a  vague  resemblance  to 
the  human  face,  the  nose  and  eyes  being  especially  prom- 
inent. Hence  the  "man  in  the  moon."  Through  even 
the  smallest  telescopes  we  see  that  the  surface  has  an  im- 
mense variety  of  detail ;  and  the  more  powerful  the  tele- 


THE    SUN,    EARTH,    AND    MOON 


FIG.  20. — Mountainous  Surface  of  the  Moon. 


SURFACE    OF    THE    MOON  125 

scope  the  more  details  we  see.  The  first  thing  to  strike 
us  on  a  telescopic  examination  will  be  the  elevations,  or 
mountains  as  they  are  commonly  called.  These  are  best 
seen  about  the  time  of  the  first  quarter,  because  they  then 
cast  shadows.  At  full  moon  they  cannot  be  so  well  made 
out,  because  we  are  looking  straight  down  and  see  every- 
thing illuminated.  Although  these  elevations  and  de- 
pressions are  called  mountains  they  are  different  in 
form  from  the  ordinary  mountains  of  the  earth. 
There  is,  however,  an  almost  exact  resemblance  be- 
tween them  and  the  craters  of  our  great  volcanoes. 
A  very  common  form  is  that  of  a  circular  fort,  one 
or  more  miles  in  diameter,  with  walls  which  may  be 
thousands  of  feet  high.  The  inside  of  this  fort  may 
be  saucer  shaped,  a  large  portion  of  the  surface  being 
flat.  At  first  quarter  we  can  see  the  shadow  of  the  walls 
cast  upon  the  interior  flat  surface.  In  the  centre  a  little 
cone  is  frequently  seen.  The  interior  surface  is  by  no 
means  perfectly  flat  and  smooth.  The  higher  power  the 
more  details  we  shall  see.  Just  what  these  consist  of  it  is 
impossible  to  say ;  they  may  be  solid  rock  or  they  may  be 
piles  of  loose  stone.  As  we  can  see  no  object  on  the  moon, 
even  with  the  most  powerful  telescope,  unless  it  is  more 
than  a  hundred  feet  in  diameter,  we  cannot  say  what  the 
exact  nature  of  the  surface  is  in  its  minutest  portions. 

The  early  observers  with  the  telescope  supposed  that 
the  dark  portions  were  seas  and  the  brighter  portions 
continents.  This  notion  was  founded  on  the  fact  that  the 
darker  portions  looked  smoother  than  the  others.  Names 
were  therefore  given  to  these  supposed  oceans,  such 


126       THE    SUN,    EARTH,    AND    MOON 

as  Mare  Procellarum,  the  Sea  of  Storms ;  Mare  Serenita- 
tisy  the  Sea  of  Calms,  etc.  These  names,  fanciful  though 
they  be,  are  still  retained  to  designate  the  large  dark 
regions  on  the  moon.  A  very  slight  improvement  in  the 
telescope,  however,  showed  that  the  idea  of  these  dark 
regions  being  oceans  was  an  illusion.  They  are  all  cov- 
ered with  inequalities,  proving  that  they  must  be  com- 
posed of  solid  matter.  The  difference  of  aspect  arises 
from  the  lighter  or  darker  shade  of  the  materials  which 
compose  the  lunar  surface.  These  are  distributed  over 
the  surface  of  the  moon  in  a  very  curious  way.  One  of 
the  most  remarkable  features  is  the  long  bright  lines 
which  radiate  from  certain  points  on  the  moon.  A  very 
low  telescopic  power  will  show  the  most  remarkable  of 
these;  a  good  eye  might  even  perceive  it  without  a  tele- 
scope. On  the  southern  part  of  the  moon's  hemisphere, 
as  we  see  it,  is  a  large  spot  or  region  known  as  Tycho, 
and  from  this  radiate  a  number  of  these  bright  streaks. 
The  appearance  is  as  if  the  moon  had  been  cracked  and 
the  cracks  filled  up  with  melted  white  matter. 

Whether  we  accept  this  view  or  not,  it  is  impossible  to 
examine  the  surface  of  the  moon  without  the  conviction 
that  in  some  former  age  it  was  the  seat  of  great  volcanic 
activity.  In  the  centre  of  all  the  great  circular  moun- 
tains we  have  described  are  craters  which,  it  would  seem, 
must  have  been  those  of  volcanoes.  Indeed,  a  hundred 
years  ago  it  was  supposed  by  Sir  William  Herschel  that 
there  was  an  active  volcano  on  the  moon,  but  it  is  now 
known  that  this  appearance  is  due  to  the  light  of  the 
earth  reflected  from  a  very  bright  spot  on  the  moon's 


AIR  OR  WATER  ON  THE  MOON?       127 

surface.    It  can  be  easily  seen  about  the  time  of  the  new 
moon  with  a  telescope  of  moderate  size. 

Is  there  Air  or  Water  on  the  Moon? 

One  of  the  most  important  questions  connected  with 
the  moon  is  whether  there  is  any  air  or  water  on  its  sur- 
face. To  these  the  answer  of  science  up  to  the  present 
time  is  in  the  negative.  Of  course  this  does  not  mean 
that  there  can  absolutely  not  be  a  drop  of  moisture  nor 
the  smallest  trace  of  an  atmosphere  on  our  satellite;  all 
we  can  say  is  that  if  any  atmosphere  surrounds  the  moon 
it  is  so  rare  that  we  have  never  been  able  to  get  any  evi- 
dence of  its  existence.  If  the  latter  had  such  an  append- 
age of  even  one  hundredth  of  the  density  of  the  earth's 
atmosphere,  its  existence  would  be  made  known  to  us  by 
refraction  of  the  light  from  a  star  seen  alongside  the 
moon.  But  not  the  slightest  trace  of  any  such  refrac- 
tion can  be  discovered.  If  there  is  any  such  liquid  as 
water,  it  must  be  concealed  in  invisible  crevices,  or  dif- 
fused through  the  interior.  Were  there  any  large  sheets 
of  water  in  the  equatorial  regions  they  would  reflect  the 
light  of  the  sun  day  by  day,  and  would  thus  become 
clearly  visible.  The  water  would  also  evaporate  and  form 
more  or  less  of  an  atmosphere  of  watery  vapour. 

All  this  seems  to  settle  another  important  question; 
namely,  that  of  the  habitability  of  the  moon.  Life,  in 
the  form  in  which  it  exists  on  our  earth,  requires  water 
at  least  for  its  support,  and  in  all  its  higher  forms  air 
also.  We  can  hardly  conceive  of  a  living  thing  made  of 
mere  sand  or  other  dry  matter  such  as  forms  the  lunar 


128       THE    SUN,    EARTH,    AND    MOON 

surface.  If  we  supposed  animals  to  walk  about  on  the 
moon,  it  is  difficult  to  imagine  what  they  could  eat.  Our 
general  conclusion  must  be  that  there  is  no  life  on  the 
moon  subject  to  the  laws  which  govern  life  on  the  surface 
of  this  earth. 

The  total  absence  of  air  and  water  results  in  a  state  of 
things  on  the  moon  such  as  we  never  experience  on  the 
earth.  So  far  as  can  be  ascertained  by  the  most  careful 
examination,  not  the  slightest  change  ever  takes  place 
on  its  surface.  A  stone  lying  on  the  surface  of  the  earth 
is  continually  attacked  by  the  weather  and  in  the  course 
of  years  is  gradually  disintegrated  or  washed  away  by 
the  wind  and  water.  But  there  is  no  weather  on  the  moon, 
and  a  stone  lying  on  its  surface  might  rest  there  for  un- 
known ages  undisturbed  by  any  cause  whatever.  The 
lunar  surface  is  heated  up  when  the  sun  shines  on  it  and 
it  cools  off  when  the  sun  has  set.  Except  for  these 
changes  of  temperature  there  is  absolutely  nothing  going 
on  over  the  whole  surface  of  the  moon,  so  far  as  we  can 
see.  A  world  which  has  no  weather  and  on  which  nothing 
ever  happens — such  is  the  moon. 

Rotation  of  the  Moon 

The  rotation  of  the  moon  on  its  axis  is  a  subject  on 
which  some  are  frequently  so  perplexed  that  we  shall 
explain  it.  Anyone  who  has  carefully  examined  this 
body  knows  that  it  always  presents  the  same  face  to  us. 
This  shows  that  it  rotates  on  its  axis  in  the  same  time 
that  it  revolves  around  the  earth.  An  idea  frequently 
entertained  is  that  this  shows  that  it  does  not  rotate  at  all, 


HOW  THE  MOON  PRODUCES  TIDES    129 


and  many  chapters  have  been  written  on  this  subject. 
The  whole  difficulty  arises  from  the  different  ideas  which 
people  have  of  motion.  In  physics  we  say  that  a  body 
does  not  rotate  when,  if  a  rod  were  passed  through  it, 
that  rod  always  maintained  the  same  direction  when  the 
body  moved  about. 
Now  let  us  sup- 
pose such  a  rod 
passed  through  the 
moon ;  then,  if  the 
latter  did  not  ro- 
tate on  its  axis  the 
rod  would  main- 
tain its  same  direc- 
tion while  the 
moon,  revolving 
around  the  earth, 
would  appear  at 
different  points  in 
its  orbit  as  we  see  it  in  Figure  21.  A  very  little  study 
of  this  figure  will  show  that  as  the  moon  went  around 
we  should  successively  see  every  part  of  its  surface 
in  succession  if  it  did  not  rotate  on  its  axis. 

How  the  Moon  Produces  the  Tides 

All  of  us  who  live  on  the  seashore  know  that  there  is 
a  rise  and  fall  of  the  ocean  which  in  the  general  average 
occurs  about  three  quarters  of  an  hour  later  every  day, 
and  which  keeps  pace  wich  the  apparent  diurnal 'motion 
of  the  moon.  That  is  to  say,  if  it  is  high  tide  to-day  when 


FIG.  21. — SJiwoing  how  the  Moon  would  Move  if 
it  did  itot  Rotate  on  its  Axis. 


130       THE    SUN,    EARTH,    AND    MOON 

the  moon  is  in  a  certain  position  in  the  heavens,  it  will  be 
high  tide  when  the  moon  is  in  or  near  that  position  day 
after  day,  month  after  month,  and  year  after  year.  We 
have  all  heard  that  the  moon  produces  these  tides  by  its 
attraction  on  the  ocean.  We  readily  understand  that 
when  the  moon  is  above  any  region  its  attraction  tends 
to  raise  the  waters  in  that  region;  but  the  circumstance 
that  most  perplexes  those  who  are  not  expert  in  the  sub- 
ject is  that  there  are  two  tides  a  day,  high  tide  occurring 
not  only  under  the  moon,  but  on  the  side  of  the  earth 
opposite  the  moon.  The  explanation  of  this  is  that  the 
moon  really  attracts  the  earth  itself  as  well  as  it  does 
the  water.  It  continually  draws  the  entire  earth  and 
everything  upon  it  toward  itself.  As  it  goes  round  the> 
earth  in  its  monthly  course,  it  thus  keeps  up  a  continual 
motion  of  the  latter.  If  it  attracted  every  part  of  the 
earth  equally,  the  ocean  included,  there  would  then  be 
no  tides,  and  everything  would  go  on  on  the  earth's  sur- 
face as  if  there  were  no  attraction  at  all.  But  as  the 
attraction  is  as  the  inverse  square  of  the  distance,  the 
moon  attracts  the  regions  of  the  earth  and  oceans  which 
are  nearest  to  it  more  than  the  average,  and  those  that 
are  farthest  from  it  less  than  the  average. 

To  show  the  effect  of  these  changes  let  A,  C,  and  H  be 
the  three  points  on  the  earth  attracted  by  the  moon. 
Since  the  moon  attracts  C  more  than  A,  it  tends  to  pull 
C  away  from  A  and  increase  the  distance  between  A  and 
C.  At  the  same  time  pulling  H  more  than  C  it  tends  to 
increase  the  distance  between  H  and  C.  If  the  whole  earth 
was  a  fluid,  the  attraction  of  the  moon  would  be  simply  to 


HOW  THE  MOON  PRODUCES  TIDES     131 

draw  this  fluid  out  into  the  form  of  an  ellipsoid,  of  which 
the  long  diameter  would  be  turned  toward  the  moon.  But 
the  earth  itself,  being  solid,  cannot  be  drawn  out  into  this 
shape,  while  the  ocean,  being  fluid,  is  thus  drawn  out. 
The  result  is  that  we  have  high  tides  at  the  two  ends  of 
the  ellipse  into  which  the  ocean  is  drawn,  and  low  tides 
in  the  mid-region. 

The  complete  explanation  of  the  subject  requires  a 
statement  of  the  laws  of  motion  which  cannot  be  made 


MOON 


FIG.  22. — How  the  Moon's  Pull  on  the  Earth  and  Ocean  Produces  Two 
Tides  in  a  Day. 

here.  I  will,  however,  remark  that  if  the  attraction  of 
the  moon  on  the  earth  were  always  in  the  same  direction, 
the  two  bodies  would  be  drawn  together  in  a  few  days. 
But  owing  to  the  revolution  of  the  moon  round  the  earth 
.the  direction  of  the  pull  is  always  changing,  so  that  the 
earth  is,  in  the  course  of  a  month,  only  drawn  about 
three  thousand  miles  from  its  mean  position  by  the 
moon's  pull. 

It  might  be  supposed  that  if  the  moon  produces  the 
tides  in  this  way  we  should  always  have  high  tide  when 
the  moon  is  on  the  meridian  and  low  tide  when  the  moon 
is  in  the  horizon.  But  such  is  not  the  case,  for  two  rea- 
sons. In  the  first  place  it  takes  time  for  the  moon  to  draw 


132       THE    SUN,    EARTH,    AND    MOON 

the  waters  out  into  the  form  of  an  ellipsoid,  and  when 
it  once  gives  them  the  motion  necessary  to  keep  this  form, 
that  motion  keeps  up  after  the  moon  has  passed  the 
meridian,  just  as  a  stone  continues  to  rise  after  it  has  left 
the  hand  or  a  wave  goes  forward  by  the  momentum  of 
the  water.  The  other  cause  is  found  in  the  interruption 
of  the  motion  by  the  great  continents.  The  tidal  wave, 
as  it  is  called,  meeting  a  continent,  spreads  out  in  one 
direction  or  the  other,  according  to  the  lay  of  the  land, 
and  may  be  a  long  time  in  passing  from  one  point  to 
another.  Thus  arise  all  sorts  of  irregularities  in  the 
tides  when  we  compare  those  in  different  places. 

The  sun  produces  a  tide  as  well  as  the  moon,  but  a 
smaller  one.  At  the  times  of  new  and  full  moon  the 
two  bodies  unite  their  forces  and  cause  the  highest  and 
lowest  tides.  These  are  familiar  to  all  dwellers  on  the 
seacoast  and  are  called  spring  tides.  About  the  time 
of  the  first  and  last  quarters  the  attraction  of  the  sun 
opposes  that  of  the  moon  and  the  tides  do  not  rise  so 
high  or  fall  so  low,  and  these  are  called  neap  tides. 


V 
ECLIPSES    OF    THE    MOON 

THE  reader  is  doubtless  aware  that  an  eclipse  of  the 
moon  is  caused  by  that  body  entering  the  shadow  of  the 
earth,  and  that  an  eclipse  of  the  sun  is  caused  by  the 
moon  passing  between  us  and  the  sun.  Taking  this 
knowledge  for  granted,  we  shall  explain  the  more  inter- 
esting features  of  these  phenomena  and  the  laws  of  their 
recurrence. 

The  first  question  to  be  considered  is:  Why  is  there 
not  an  eclipse  of  the  moon  at  every  full  moon,  since  the 
earth's  shadow  must  always  be  in  its  place  opposite  the 


FIG.  23. —  TJie  Moon  in  tJie  SJiadow  of  the  Earth. 

sun?  The  answer  is  that  the  moon  commonly  passes 
either  above  or  below  the  shadow  of  the  earth,  and  so  fails 
to  be  eclipsed.  This,  again,  arises  from  the  fact  that 
the  orbit  of  the  moon  has  a  small  inclination,  about  five 
degrees,  to  the  plane  of  the  ecliptic,  in  which  the  earth 
moves,  and  in  which  the  centre  of  the  shadow  always  lies. 
Returning  to  our  former  thought  of  the  ecliptic  being 


THE    SUN,    EARTH,    AND    MOON 

marked  out  on  the  celestial  sphere,  let  us  suppose  that 
we  also  mark  out  the  orbit  of  the  moon  during  the  course 
of  its  monthly  period.  We  should  then  find  the  orbit  of 
the  moon  crossing  that  of  the  sun  in  two  opposite  points, 
at  the  very  small  angle  of  five  degrees.  These  points  of 
crossing  are  called  nodes.  At  one  node  the  moon  passes 
from  below,  or  south  of  the  ecliptic,  to  the  north  of  it. 
This  is  called  the  ascending  node.  At  the  other  the 
moon  passes  from  north  to  south  of  the  ecliptic.  This  is 
called  the  descending  node.  The  terms  ascending  and 
descending  are  applied  to  the  node,  because  to  us  in  the 
northern  hemisphere,  the  north  side  of  the  ecliptic  and 
equator  seem  to  be  above  the  south  side. 

At  the  points  halfway  between  the  nodes  the  centre 
of  the  moon  is  above  the  ecliptic  by  about  one  twelfth  its 
distance  from  us,  that  is,  by  about  twenty  thousand  miles. 
The  sun  being  larger  than  the  earth,  the  shadow  of  the 
latter  gradually  grows  smaller  away  from  the  earth.  At 
1  the  distance  of  the  moon  its  diameter  is  about  three 
"fourths  that  of  the  earth,  that  is  about  six  thousand 
miles.  Its  centre  being  in  the  plane  of  the  ecliptic,  it 
extends  only  about  three  thousand  miles  above  and  below 
that  plane.  Hence  it  is  that  the  moon  will  pass  through 
it  only  when  near  the  nodes. 

Eclipse  Seasons 

The  line  joining  the  sun  and  moon  of  course  turns 
round  as  the  earth  moves  around  the  sun.  It  therefore 
crosses  the  moon's  nodes  twice  in  the  course  of  a  year. 
That  is  to  say  if  we  suppose  the  nodes  to  be  marked  in  the 


ECLIPSE    SEASONS  135 

sky,  the  ascending  node  at  one  point,  and  the  descending 
node  at  the  opposite  point,  then  the  sun  will  appear  to 
us  to  pass  each  of  these  points  in  the  course  of  a  year. 
While  the  sun  is  passing  one  node  the  shadow  of  the 
earth  will  seem  to  be  passing  the  other.  It  is  only  near 
these  two  times  of  the  year  that  an  eclipse  of  the  sun  or 
moon  can  occur.  We  may  therefore  call  them  eclipse 
seasons.  They  commonly  last  about  a  month;  that  is 
to  say  it  is  generally  about  a  month  from  the  time  when 
the  sun  gets  near  enough  to  a  node  to  allow  of  an 
eclipse  until  the  time  when  it  is  too  far  past  for  an 
eclipse  to  occur.  In  1901  the  seasons  were  May  and 
November. 

If  the  moon's  node  stayed  in  the  same  place  in  the  sky, 
eclipses  would  occur  only  some  time  during  these  two 
months.  But,  owing  to  the  attraction  of  the  sun  on  the 
earth  and  moon,  the  position  of  the  nodes  is  continually 
changing  in  a  direction  opposite  that  of  the  motion  of 
the  two  bodies.  Each  node  makes  a  complete  revolution 
around  the  celestial  sphere  in  eighteen  years  and  seven 
months.  Hence  in  this  same  period  the  eclipse  seasons 
will  course  all  through  the  year.  On  an  average  they 
occur  about  nineteen  days  earlier  every  year  than  they 
did  the  year  before.  Thus  it  happens  that  in  1903  one 
season  occurs  in  March  and  April  and  the  other  season  in 
September  and  October.  The  change  will  keep  going  on 
until,  in  the  year  1910,  the  season  which  in  1901  was  in 
May  will  have  gotten  back  to  November,  while  the  No- 
vember one  will  have  gotten  back  to  May,  each  having 
passed  through  all  the  intermediate  months,  and  the  two 


136       THE    SUN,    EARTH,    AND    MOON 

having  changed  places.     By  1919  each  will  have  made 
an  entire  revolution  through  the  year. 

Let  us  imagine  ourselves  to  be  looking  at  the  sun  and 
earth  from  the  moon  when  the  latter  is  about  to  enter  the 
earth's  shadow.  The  earth,  looking  much  larger  than 
the  sun,  will  be  seen  to  approach  it,  and  at  length  will 
begin  to  impinge  on  its  disk  and  cut  off  a -part  of  its 
light.  The  region  within  which  this  will  occur  is  called 
the  penumbra,  and  it  is  shown  outside  the  shadow  in  the 
figure.  So  long  as  the  moon  is  only  in  this  region,  an 


FIG.  24. — Passage  of  the  Moon  through  the  Earths  Shadow. 

ordinary  observer  would  not  notice  any  diminution  in  its 
light,  although  such  a  diminution  could  be  detected  by 
exact  photometric  measurements.  The  moon  is  not  said 
to  be  eclipsed  until  it  begins  to  enter  into  the  actual 
shadow,  where  the  whole  direct  light  of  the  sun  is  cut  off. 

How  an  Eclipse  of  the  Moon  Looks 

If  we  watch  the  moon  when  an  eclipse  is  about  to  be- 
gin, we  shall  see  a  small  portion  of  her  eastern  edge  grad- 
ually grow  dim  and  finally  disappear.  As  the  moon 
advances  in  her  orbit,  more  and  more  of  her  face  thus 
disappears  from  view  by  entering  into  the  shadow.  If, 
however,  we  look  very  carefully,  we  shall  see  that  the  part 


HOW  AN  ECLIPSE  OF  MOON  LOOKS     137 

immersed  in  the  shadow  has  not  entirely  disappeared,  but 
shines  with  a  very  faint  light.  If  the  whole  body  of  the 
moon  enters  into  the  shadow,  the  eclipse  is  said  to  be 
total ;  if  only  a  portion  of  her  body  dips  into  the  shadow, 
it  is  called  partial.  If  the  eclipse  is  total,  the  light  which 
illuminates  the  eclipsed  moon  will  be  very  plainly  seen, 
because  it  is  not  drowned  out  by  the  dazzling  light  of  the 
uneclipsed  portion.  This  light  is  of  a  dingy  red  colour, 
and  arises  from  the  refraction  of  the  earth's  atmosphere, 
which  was  described  in  a  former  chapter.  In  consequence 
of  this,  those  rays  of  the  sun  which  just  graze  the  earth, 
or  pass  within  a  short  distance  of  its  surface,  are  bent  out 
of  their  course  and  thrown  into  the  shadow  by  refraction. 
Thus  they  fill  the  shadow  and  fall  on  the  moon.  The  red 
colour  is  due  to  the  same  cause  that  makes  the  sun  appear 
red  at  sunset,  namely,  the  absorption  of  the  green  and 
blue  rays  by  the  atmosphere,  which  lets  the  red  rays  pass. 

Two  or  three  eclipses  of  the  moon  occur  every  year,  of 
which  one,  at  least,  is  nearly  always  total.  But,  of 
course,  the  eclipse  will  be  visible  only  in  that  hemisphere 
of  the  earth  on  which  the  moon  is  shining  at  the  time. 

When  the  moon  is  eclipsed  an  observer  on  that  body 
would  see  an  eclipse  of  the  sun  by  the  earth.  The  cause 
of  the  phenomenon  we  have  described  would  then  be  plain 
enough  to  him.  The  apparent  size  of  the  earth  would 
be  much  larger  than  that  of  the  moon  as  we  see  it.  Its 
diameter  would  be  between  three  and  four  times  that  of 
the  sun.  At  first  this  immense  body  would  be  invisible 
when  it  approached  the  sun.  What  the  observer  would 
see  would  be  the  cutting  off  of  the  light  of  the  sun  by  the 


138       THE    SUN,    EARTH,    AND    MOON 

advancing  but  invisible  earth.  When  the  latter  had 
nearly  covered  the  sun,  its  whole  outline  would  be  shown 
to  liim  by  a  red  light  surrounding  it,  caused  by  the  re- 
fraction of  the  earth's  atmosphere.  Finally,  when  the 
last  trace  of  true  sunlight  had  disappeared,  nothing 
would  be  visible  but  this  ring  of  bright  red  light  having 
inside  of  it  the  black  but  otherwise  invisible  body  of  the 
earth. 

The  circumstances  of  an  eclipse  of  the  moon  are  quite 
different  from  those  of  a  solar  eclipse,  to  be  described  in 
the  next  chapter.  It  can  aways  be  seen  at  the  same  in- 
stant over  the  whole  hemisphere  of  the  earth  on  which 
the  moon  is  shining  at  the  time.  A  curious  phenomenon 
occurs  when  the  moon  rises  totally  eclipsed.  Then  we 
may  see  it  on  one  horizon,  say  the  eastern  one,  while  the 
sun  is  still  visible  on  the  western  horizon.  The  explana- 
tion of  this  seeming  paradox  is  that  both  bodies  are  realty 
below  the  horizon,  but  are  so  elevated  by  refraction  that 
we  can  see  them  at  the  same  time. 


VI 

ECLIPSES  OP  THE  SUN 

IF  the  moon  moved  exactly  in  the  plane  of  the  ecliptic 
she  would  pass  over  the  face  of  the  sun  at  every  new 
moon.  But,  owing  to  the  inclination  of  her  orbit,  as  de- 
scribed in  the  preceding  chapter,  she  will  actually  do  so 
only  when  the  direction  of  the  sun  happens  to  be  near  one 
of  the  moon's  nodes.  When  this  is  the  case  we  may  see 
an  eclipse  of  the  sun  if  we  are  only  on  the  right  part  of 
the  earth. 

Supposing  the  moon  to  pass  over  the  sun,  the  first 
question  is  whether  it  can  wholly  hide  the  sun  from  our 
eyes.  This  depends  not  on  the  actual  size  of  the  two 


FIG.  25.—  77ie  Sliadow  of  t7ie  Moon  TJirown  on  tJie  Earth  during  a  Total 
Eclipse  of  ike  Sun. 

bodies  but  on  their  apparent  size.  We  know  that  the  sun 
has  about  four  hundred  times  the  diameter  of  the  moon. 
But  it  is  also  four  hundred  times  as  far  from  us  as  the 
moon.  The  curious  result  of  this  is  that  the  two  bodies 
appear  of  nearly  the  same  size  to  our  eyes.  Sometimes 


140       THE    SUN,   EARTH,   AND    MOON 

the  moon  appears  a  little  the  larger,  and  sometimes  the 
sun.  In  the  former  case  the  moon  may  entirely  hide  the 
sun ;  in  the  latter  case  she  cannot  do  so. 

One  important  difference  between  an  eclipse  of  the 
moon  and  of  the  sun  is  that  the  former  is  always  the  same 
wherever  it  is  visible,  while  an  eclipse  of  the  sun  depends 
upon  the  position  of  the  observer.  The  most  interesting 
eclipses  are  those  in  which  the  centre  of  the  moon  passes 
exactly  over  that  of  the  sun.  These  are  called  central 


FIG.  26. — TJie  Moon  Passing   Centrally  over  the  Sun  during  an  Annular 

Eclipse. 

eclipses.  To  see  one,  the  observer  must  station  himself 
at  a  point  through  which  the  line  joining  the  centres 
shall  pass.  Then  if  the  apparent  size  of  the  moon  ex- 
ceeds that  of  the  sun,  the  former  will  completely  hide  the 
sun  from  view.  The  eclipse  is  then  said  to  be  total. 

If  the  sun  appears  the  larger,  a  ring  of  its  light  will 
surround  the  dark  body  of  the  moon  at  the  moment  of 
central  eclipse.  The  latter  is  then  called  annular  (Latin 
annulus,  a  ring). 

The  line  of  centres  of  the  two  bodies  sweeps  along  the 
surface  of  the  earth,  and  its  course  may  be  shown  by  a 
line  marked  on  a  map.  Such  maps,  showing  the  regions 


BEAUTY  OF  A  TOTAL  ECLIPSE         141 

and  lines  of  eclipses  are  published  in  the  astronomical 
ephemerides.  An  eclipse  may  be  total  or  annular  in  a 
region  a  few  miles  north  or  south  of  this  central  line,  but 
never  for  so  far  as  one  hundred  miles.  Outside  this 
limit  an  observer  will  see  only  a  partial  eclipse,  that  is, 
one  in  which  the  moon  partly  covers  the  sun.  In  yet 
more  distant  regions  of  the  earth  there  will  be  no  eclipse 

at  all. 

Beauty  of  a  Total  Eclipse 

A  total  eclipse  is  one  of  the  most  impressive  sights  that 
nature  offers  to  the  eye  of  man.  To  see  it  to  the  best 
advantage  one  should  be  in  an  elevated  position  com- 
manding the  widest  possible  view  of  the  surrounding 
country,  especially  in  the  direction  from  which  the 
shadow  of  the  moon  is  to  come.  The  first  indication  of 
anything  unusual  is  to  be  seen,  not  on  the  earth  or  in  the 
air,  but  on  the  disk  of  the  sun.  At  the  predicted  moment 
a  little  notch  will  be  seen  to  form  somewhere  on  the  west- 
ern edge  of  the  sun's  outline.  It  increases  minute  by 
minute,  gradually  eating  away,  as  it  were,  the  visible 
sun.  No  wonder  that  imperfectly  civilised  people,  when 
they  saw  the  great  luminary  thus  diminishing  in  size, 
fancied  that  a  dragon  was  devouring  its  substance. 

For  some  time,  perhaps  an  hour,  nothing  will  be 
noticed  but  the  continued  progress  of  the  advancing 
moon.  It  will  be  interesting  if,  during  this  time,  the  ob- 
server is  in  the  neighbourhood  of  a  tree  that  will  permit 
the  sun's  rays  to  reach  the  ground  through  the  small 
openings  in  its  foliage.  The  little  images  of  the  sun 
which  form  here  and  there  on  the  ground  will  then  have 


THE    SUN,    EARTH,    AND    MOON 

the  form  of  the  partially  eclipsed  sun.  Soon  the  latter 
appears  as  the  new  moon,  only  instead  of  increasing,  the 
crescent  form  grows  thinner  minute  by  minute.  Even 
then,  so  well  has  the  eye  accommodated  itself  to  the 
diminishing  light,  there  may  be  little  noticeable  darkness 
until  the  crescent  has  grown  very  thin.  If  the  observer 
has  a  telescope  with  a  dark  glass  for  viewing  the  sun,  he 
will  now  have  an  excellent  opportunity  of  seeing  the 
mountains  on  the  moon.  The  unbroken  limb  of  the  sun 
will  keep  its  usual  soft  and  uniform  outline.  But  the 
inside  of  the  crescent,  the  edge  of  which  is  formed  by 
the  surface  of  the  moon,  will  be  rough  and  jagged  in 
outline. 

As  the  crescent  is  about  to  disappear  the  advancing 
mountains  on  the  rugged  surface  of  the  moon  will  reach 
the  sun's  edge,  leaving  nothing  of  the  latter  but  a  row  of 
broken  fragments  or  points  of  light,  shining  between 
the  hollows  on  the  lunar  surface.  They  last  but  a  second 
or  two  and  then  vanish. 

Now  is  seen  the  glory  of  the  spectacle.  The  sky  is 
clear  and  the  sun  in  mid-heaven,  and  yet  no  sun  is  visible. 
Where  the  latter  ought  to  be  the  densely  black  globe  of 
the  moon  hangs,  as  it  were,  in  mid-air.  It  is  surrounded 
by  an  effulgence  radiating  a  saintly  glory.  This  is  the 
sun's  corona,  already  mentioned  in  our  chapter  on  the 
sun.  Though  bright  enough  to  the  unaided  vision,  it  is 
seen  to  the  best  advantage  with  a  telescope  of  very  low 
magnifying  power.  Even  a  common  opera  glass  may 
suffice.  With  a  telescope  of  high  power  only  a  portion 
of  the  corona  is  visible,  and  thus  the  finest  part  of  the 


ANCIENT    ECLIPSES  143 

effect  is  lost.  A  common  spy-glass,  magnifying  ten  or 
twelve  times,  is  better,  so  far  as  effect  is  concerned,  than 
the  largest  telescope.  Such  an  instrument  will  show  not 
only  the  corona  itself  but  the  so-called  "prominences"- 
fantastic  cloud-like  forms  of  rosy  colour  rising  here  and 
there,  seemingly  from  the  dark  body  of  the  moon. 

Ancient  Eclipses 

It  is  remarkable  that  though  the  ancients  were  familiar 
with  the  fact  of  eclipses,  and  the  more  enlightened  of 
them  perfectly  understood  their  causes,  some  even  the 
laws  of  their  recurrence,  there  are  very  few  actual  ac- 
counts of  these  phenomena  in  the  writings  of  the  ancient 
historians.  The  old  Chinese  annals  now  and  then  record 
the  fact  that  an  eclipse  of  the  sun  occurred  at  a  certain 
time  in  some  province  or  near  some  city  of  the  empire. 
But  no  particulars  are  given.  Quite  recently  the  Assyri- 
ologists  have  deciphered  from  ancient  tablets  a  statement 
that  an  eclipse  of  the  sun  was  seen  at  Nineveh,  B.  C.  763, 
June  15.  Our  astronomical  tables  show  that  there  actu- 
ally was  a  total  eclipse  of  the  sun  on  this  day,  during 
which  the  shadow  passed  a  hundred  miles  or  so  north  of 
Nineveh. 

Perhaps  the  most  celebrated  of  the  ancient  eclipses, 
and  the  one  that  has  given  rise  to  most  discussion,  is  that 
known  as  the  eclipse  of  Thales.  Its  principal  historical 
basis  is  a  statement  of  Herodotus  that  in  a  battle  between 
the  Lydians  and  the  Medes  the  day  was  suddenly  turned 
into  night.  The  armies  thereupon  ceased  battle  and  were 
more  eager  to  come  to  terms  of  peace  with  each  other.  It 


144       THE    SUN,    EARTH,    AND    MOON 

is  added  that  Thales,  the  Milesian,  had  predicted  to  the 
lonians  this  change  of  day,  even  the  very  year  in  which 
it  should  occur.  Our  astronomical  tables  show  that  there 
actually  was  a  total  eclipse  of  the  sun  in  the  year  B.  C. 
585,  which  was  near  enough  to  the  time  of  the  battle  to 
be  the  one  alluded  to,  but  it  is  now  known  that  the  path 
of  the  shadow  did  not  quite  reach  the  seat  of  hostilities 
till  after  sunset.  Some  doubt  therefore  still  rests  on  the 
subject. 

Prediction  of  Eclipses 

There  is  a  curious  law  of  the  recurrence  of  eclipses 
which  has  been  known  from  ancient  times.  It  is  based  on 
the  fact  that  the  sun  and  moon  return  to  nearly  the  same 
positions,  relative  to  the  node  and  perigee  of  the  moon's 
orbit,  after  a  period  of  six  thousand  five  hundred  and 
eighty-five  days  eight  hours,  or  eighteen  years  and 
twelve  days.  This  period  is  called  the  Saros.  Eclipses 
of  every  sort  repeat  themselves  at  the  end  of  a  Saros. 
For  example,  the  eclipse  of  May,  1900,  may  be  regarded 
as  a  repetition  of  those  which  occurred  in  the  years  1846, 
1864,  and  1882.  But  when  such  an  eclipse  recurs  it  is 
not  visible  in  the  same  part  of  the  earth,  because  of  the 
excess  of  eight  hours  in  the  period.  During  this  eight 
hours  the  earth  performs  one  third  of  a  rotation  on  its 
axis,  which  brings  a  different  region  under  the  sun.  Each 
eclipse  is  visible  in  a  region  about  one  third  of  the  way 
round  the  world,  or  one  hundred  and  twenty  degrees  of 
longitude,  west  of  where  it  occurred  before.  Only  after 
three  periods  will  the  recurrence  be  near  the  same  region. 
But  in  the  meantime  the  moon's  line  of  motion  will  have 


THE    SUN'S    APPENDAGES  145 

changed  so  that  the  path  of  its  shadow  will  pass  farther 
north  or  south  than  before. 

There  are  two  series  of  eclipses  remarkable  for  the 
long  duration  of  the  total  phase.  To  one  of  these  the 
eclipse  of  1868,  hereafter  mentioned,  belongs.  This  re- 
curred in  1886,  and  will  recur  again  in  1904.  Unfortu- 
nately, at  the  first  recurrence,  the  shadow  was  cast  almost 
entirely  on  the  Atlantic  and  Pacific  Oceans,  so  that  it 
was  not  favourable  for  observation  by  astronomers.  That 
of  1904,  September  9,  will  be  yet  more  unfortunate  for 
us,  because  the  shadow  will  pass  only  over  the  Pacific 
Ocean.  Possibly,  however,  it  may  touch  some  island 
where  observations  may  be  made.  The  recurrence  of 
1922,  September  1,  will  be  visible  in  northern  Australia, 
where  the  duration  of  totality  will  be  about  four  minutes. 

To  the  other  and  yet  more  remarkable  series  belonged 
the  eclipse  of  May  7,  1883,  and  that  of  May  11,  1901. 
At  the  successive  recurrences  of  this  eclipse  the  duration 
of  totality  will  be  longer  and  longer  through  the  twenti- 
eth century.  In  1937,  1955,  and  1973  it  will  exceed 
seven  minutes,  so  that  so  far  as  duration  is  concerned,  our 
successors  will  see  eclipses  more  remarkable  than  any 
their  ancestors  have  enjoyed  for  many  centuries. 

The  Sun's  Appendages 

About  1863-64  the  spectroscope  began  to  be  applied 
to  researches  on  the  heavenly  bodies.  Mr.  (now  Sir 
William)  Huggins,  of  London,  was  a  pioneer  in  observ- 
ing the  spectra  of  the  stars  and  nebulae.  For  several 
years  it  did  not  seem  that  much  was  to  be  learned  in  this 


146       THE    SUN,    EARTH,    AND    MOON 

way  about  the  sun.  The  year  1868  at  length  arrived. 
On  August  eighteenth  there  was  to  be  a  remarkable  total 
eclipse  of  the  sun,  visible  in  India.  The  shadow  was  one 
hundred  and  forty  miles  broad ;  the  duration  of  the  total 
phase  was  more  than  six  minutes.  The  French  sent  Mr. 
Janssen,  one  of  their  leading  spectroscopists,  to  observe 
the  eclipse  in  India  and  see  what  he  could  find  out.  Won- 
derful was  his  report.  The  red  prominences  which  had 
perplexed  scientists  for  two  centuries  were  found  to  be 
immense  masses  of  glowing  hydrogen,  rising  here  and 
there  from  various  parts  of  the  sun,  of  a  size  compared 
with  which  our  earth  was  a  mere  speck.  This  was  not 
all.  After  the  sunlight  reappeared,  Janssen  began  to 
watch  these  objects  in  his  spectroscope.  He  followed 
them  as  more  and  more  of  the  sun  came  out,  and  con- 
tinued to  see  them  until  after  the  eclipse  was  over.  They 
could  be  observed  at  any  time  when  the  air  was  sufficiently 
clear  and  the  sun  high  in  the  sky. 

By  a  singular  coincidence  this  same  discovery  was 
made  independently  in  London  without  any  eclipse.  Mr. 
J.  Norman  Lockyer  was  then  rising  into  prominence  as 
an  enthusiastic  worker  with  the  spectroscope.  It  oc- 
curred independently  to  him  and  to  Mr.  Huggins  that 
the  heat  in  the  neighbourhood  of  the  sun  was  so  intense 
that  any  matter  that  existed  there  would  probably  take 
the  form  of  a  gas  shining  by  its  own  light.  Both  of 
these  investigators  endeavoured  to  get  a  sight  of  the 
prominences  in  this  way;  but  it  was  not  until  October 
twentieth,  two  months  after  the  Indian  eclipse,  that  Mr. 
Lockyer  succeeded  in  having  an  instrument  of  sufficient 


THE    SUN'S    APPENDAGES  147 

power  completed.  Then,  at  the  first  opportunity,  he 
found  that  he  could  see  the  prominences  without  an 
eclipse ! 

At  that  time  communication  with  India  was  by  mail, 
so  that  for  the  news  of  Mr.  Janssen's  discovery  astrono- 
mers had  to  wait  until  a  ship  arrived.  By  a  singular 
coincidence  his  report  and  Mr.  Lockyer's  communication 
announcing  his  own  discovery  reached  the  French  Acad- 
emy of  Sciences  at  the  same  meeting.  This  eminent  body, 
with  pardonable  enthusiasm,  caused  a  medal  to  be  struck 
in  commemoration  of  the  new  method  of  research,  in 
which  the  profiles  of  Lockyer  and  Janssen  appeared  to- 
gether as  co-discoverers.  Since  that  time  the  promi- 
nences are  regularly  mapped  out  from  da}'  to  day  by 
spectroscopic  observers  in  various  parts  of  the  world. 

The  greatest  beauty  of  a  total  eclipse  is  due  to  the 
sun's  corona.  The  exact  nature  of  this  appendage  is 
still  in  doubt.  Indeed,  until  photography  was  called  to 
the  aid  of  the  astronomer  its  structure  was  unknown.  It 
was  described  by  observers  simply  as  a  soft  light  sur- 
rounding the  sun ;  but  when  it  is  photographed  and  care- 
fully examined  it  is  found  to  be  of  a  radial,  hairy 
structure  which  the  reader  can  easily  see  from  the  fron- 
tispiece of  the  book.  It  extends  out  farthest  in  the 
direction  of  the  sun's  equator  and  least  at  the  poles.  The 
rays  which  chance  to  be  exactly  at  the  poles  go  straight 
out  from  the  sun.  But  those  on  each  side  are  found  to 
curve  toward  the  equator,  while  farther  from  the  equator 
they  are  lost  in  the  more  powerful  effulgence  going  out 
from  the  region  of  the  solar  spots.  Near  the  poles  the 


148       THE    SUN,    EARTH,    AND    MOON 

forms  are  remarkably  like  those  which  iron  filings  assume 
when  scattered  on  paper  above  a  magnet.  It  is  therefore 
a  question  whether  there  is  not  here  something  in  the  na- 
ture of  a  magnetic  force.  But  in  the  region  called  the 
sun's  equator  this  analogy  ceases  to  hold.  In  describing 
the  sun  we  mentioned  the  much  greater  activity  in  the 
regions  of  greater  spottedness  than  elsewhere.  It  now 
seems  as  if  the  forces  which  throw  out  the  corona  are 
also  greatest  where  the  sun's  activity  is  greatest. 

The  probability  now  seems  to  be  that  the  corona  is 
composed  of  matter  thrown  up  from  the  sun,  and  kept 
from  falling  back  again  by  the  repulsion  of  the  solar 
rays,  and  that  it  bears  a  certain  resemblance  to  the  tail 
of  a  comet. 

A  very  important  question  is  whether  the  corona  shines 
mostly  by  reflected  light,  or  by  its  own  light,  due  to  the 
high  temperature  which  it  must  have  so  near  the  sun. 
No  doubt  its  light  arises  from  both  sources,  but  it  is  not 
yet  known  in  what  proportion.  The  fact  is  that  its 
spectrum  shows  some  bright  lines.  These  can  be  due 
only  to  the  light  of  the  matter  itself.  Some  observers 
have  supposed  that  they  also  saw  dark  lines  in  the  spec- 
trum. This,  however,  has  not  been  proved.  On  the  whole 
the  probability  seems  to  be  that  the  corona  shines  mostly 
by  its  own  light. 


PART    IV 
THE  PLANETS  AND  THEIR  SATELLITES 


1 

ORBITS  AND  ASPECTS  OF  THE  PLANETS 

THE  orbits  in  which  the  planets  revolve 'around  their 
central  luminary  are  in  strictness  ellipses,  or  slightly 
flattened  circles.  But  the  flattening  is  so  slight  that  the 
eye  would  not  notice  it  without  measurement.  The  sun 
is  not  in  the  centre  of  the  ellipse  but  in  a  focus,  which 
in  some  cases  is  displaced  from  the  centre  by  an  amount 
that  the  eye  can  readily  perceive.  This  displacement 
measures  the  eccentricity  of  the  ellipse,  which  is  much 
greater  than  the  flattening.  For  example,  in  the  case 
of  Mercury,  which  moves  in  a  very  eccentric  orbit,  the 
flattening  is  only  one  fiftieth;  that  is,  if  we  represent 
the  greatest  diameter  of  the  orbit  by  fifty,  the  least 
diameter  will  be  forty-nine.  But  the  distance  of  the 
sun  from  the  centre  of  the  orbit  is  ten  on  the  same 
scale. 

To  show  this  we  give  a  diagram  of  the  orbits  of  the 
•inner  group  of  planets  showing  quite  nearly  their  forms 
and  respective  locations.  A  simple  glance  will  show  that 
the  orbits  are  much  nearer  together  at  some  points  than 
at  others. 

In  explaining  the  various  aspects  and  motions,  real 
and  apparent,  of  the  planets  a  number  of  technical  ex- 
pressions are  used  which  we  shall  explain. 

Inferior  planets  are  those  whose  orbits  lie  within  the 


152    PLANETS  AND  THEIR  SATELLITES 

orbit  of  the  earth.  This  class  comprises  only  Mercury 
and  Venus. 

Superior  planets  are  those  whose  orbits  lie  without 
that  of  the  earth.  These  comprise  Mars,  the  minor 
planets  or  asteroids,  and  all  four  of  the  outer  group  of 
major  planets. 

When  a  planet  seems  to  us  to  pass  by  the  sun,  and  so 


FIG.  27. — Orbits  of  the  four  Inner  Planets. 

is  seen  as  if  alongside  of  it,  it  is  said  to  be  in  conjunction 
with  the  sun. 

An  inferior  conjunction  is  one  in  which  the  planet  is 
between  us  and  the  sun. 

A  superior  conjunction  is  one  in  which  the  planet  is 
beyond  the  sun. 


ORBITS  AND  ASPECTS  OF  PLANETS    153 

A  little  consideration  will  show  that  a  superior  planet 
can  never  be  in  inferior  conduction,  but  an  inferior  planet 
has  both  kinds  of  conjunction. 

A  planet  is  said  to  be  in  opposition  when  it  is  in  the 
opposite  direction  from  the  sun.  It  then  rises  at  sunset, 
and  vice  versa.  Of  course,  an  inferior  planet  can  never 
be  in  opposition. 

The  perihelion  of  an  orbit  is  that  point  of  it  which  is 
nearest  the  sun ;  the  aphelion  its  most  distant  point  from 
the  sun. 

As  the  inferior  planets,  Mercury  and  Venus,  perform 
their  revolutions  they  seem  to  us  to  swing  from  one  side 
of  the  sun  to  the  other.  Their  apparent  distance  from 
the  sun  at  any  time  is  called  their  elongation. 

The  greatest  elongation  of  Mercury  is  generally  about 
twenty-five  degrees,  being  sometimes  more  and  sometimes 
less,  owing  to  the  great  eccentricity  of  the  orbit  of  this 
planet.  The  greatest  elongation  of  Venus  is  almost 
forty -five  degrees. 

When  the  elongation  of  one  of  these  planets  is  east 
from  the  sun  we  may  see  it  in  the  west  after  sunset; 
when  west  we  may  see  it  in  the  east  in  the  morning  sky. 
As  neither  of  them  ever  wanders  from  the  sun  farther 
than  the  distances  we  have  stated,  it  follows  that  a  planet 
seen  in  the  east  in  the  evening,  or  in  the  west  in  the  morn- 
ing, cannot  be  either  Mercury  or  Venus. 

No  two  orbits  of  the  planets  lie  exactly  in  the  same 
plane.  That  is,  if  we  regard  any  one  orbit  as  horizontal, 
all  the  others  will  be  tipped  by  small  amounts  toward  one 
side  or  the  other.  Astronomers  find  it  convenient  to  take 


154    PLANETS  AND  THEIR  SATELLITES 

the  orbit  of  the  earth,  or  the  ecliptic,  as  the  horizontal  or 
standard  one.  As  each  orbit  is  centred  on  the  sun  it 
will  have  two  opposite  points  which  lie  on  the  same  hori- 
zontal plane  as  the  earth's  orbit.  More  exactly,  these 
are  the  points  at  which  the  orbit  intersects  the  plane  of 
the  ecliptic.  They  are  called  nodes. 

The  angle  by  which  an  orbit  is  tipped  from  the  plane 
of  the  ecliptic  is  called  its  inclination.  The  orbit  of  Mer- 
cury has  the  greatest  inclination,  more  than  6°.  The 
orbit  of  Venus  is  inclined  3°  24' ;  those  of  all  the  superior 
planets  less,  ranging  from  0°  46'  in  the  case  of  Uranus 
to  2°  30'  in  the  case  of  Saturn. 

Distances  of  the  Planets 

Leaving  out  Neptune,  the  distances  of  the  planets 
follow  very  closely  a  rule  known  as  Bode's  Law,  after  the 
astronomer  who  first  pointed  it  out.  It  is  this :  Take  the 
numbers  0,  3,  6,  12,  etc.,  doubling  each  as  we  go  along. 
Then  add  4  to  each  number,  and  we  shall  hit  very  nearly 
on  the  scale  of  distances  of  all  the  planets  except  Nep- 
tune, thus : 

Mercury,      0  +  4  =      4 ;  actual  distance    4 
Venus,      3+«4  =      7;       "  "         7 

Earth,      6  +  4  =    10;       '•  "       10 


Mars,    12  +  4=    16; 

Asteroids,    24  +  4=    28; 

Jupiter,    48  +  4  =    52; 

Saturn,    96  +  4  =  100 ; 

Uranus,  192  +  4  =  196; 

Neptune,  384  +  4  =  388; 


15 

20  to  40 

52 

95 
192 
300 


On  these  actual  distances  we  remark  that  astronomers  do 


KEPLER'S    LAWS  155 

not  use  miles  or  other  terrestrial  measures  to  express 
distances  between  the  heavenly  bodies,  for  two  reasons. 
In  the  first  place,  they  are  too  short;  to  use  them  would 
be  like  stating  the  distance  between  two  cities  in  centi- 
metres. In  the  next  place,  distances  in  the  heavens  can- 
not be  fixed  with  the  necessary  exactness  in  our  measures, 
whereas,  if  we  take  the  sun's  distance  from  the  earth  as 
the  unit  of  measure,  we  can  determine  other  distances 
between  the  planets  with  great  precision  in  terms  of  this 
measure.  So,  to  get  the  distances  of  the  planets  from 
the  sun  in  astronomical  measure,  we  have  to  divide  the 
last  numbers  of  the  preceding  table  by  ten,  or  insert  a 
decimal  point  before  the  last  figure  of  each. 

We  have  not  in  this  table  distracted  the  attention  of 
the  reader  by  using  unnecessary  decimals.  Actually,  the 
distance  of  Mercury  is  0.387,  etc. ;  we  have  simply  called 
it  0.4  and  multiplied  it  by  10  to  get  the  proportion  for 
comparing  with  Bode's  Law. 

Kepler's  Laws 

The  motions  of  the  planets  in  their  orbits  take  place 
in  accordance  with  certain  laws  laid  down  by  Kepler, 
and  therefore  known  as  Kepler's  laws.  The  first  of  these 
has  already  been  mentioned ;  the  orbits  of  the  planets  are 
ellipses,  of  which  the  sun  is  in  one  focus. 

The  second  law  is  that  the  nearer  the  planet  is  to  the 
sun  the  faster  it  moves.  With  more  mathematical  exact- 
ness, the  areas  swept  over  by  the  line  joining  the  planet 
and  sun  in  equal  times  are  all  equal. 

The  third  law  is  that  the  cubes  of  the  mean  distances 


156    PLANETS  AND  THEIR  SATELLITES 

of  the  planets  from  the  sun  are  proportional  to  the 
squares  of  their  times  of  revolution.  This  law  requires 
some  illustration.  Suppose  one  planet  to  be  four  times 
as  far  from  the  sun  as  another.  It  will  then  be  eight 
times  as  long  going  around  it.  This  number  is  reached 
by  taking  the  cube  of  four,  which  is  sixty-four,  and  then 
extracting  the  square  root,  which  is  eight. 

The  unit  of  measure  which  the  astronomer  uses  to  ex- 
press distances  in  the  solar  system  being  the  mean  dis- 
tance of  the  earth  from  the  sun,  it  follows  that  the  mean 
distances  of  the  inferior  planets  will  be  decimal  fractions, 
as  we  have  just  shown,  while  those  of  the  outer  ones  will 
vary  from  1.5  in  the  case  of  Mars  to  30  in  the  case  of 
Neptune.  If  we  take  the  cubes  of  all  these  distances  and 
extract  their  square  roots  we  shall  have  the  times  of  the 
revolution  of  the  planets,  expressed  in  years. 

It  will  be  seen  that  the  outer  planets  are  longer  in 
getting  around  their  orbits,  not  only  because  they  have 
farther  to  go,  but  because  they  actually  move  more 
slowly.  If,  as  in  the  case  first  supposed,  the  outer  planet 
is  four  times  as  far  from  the  sun,  it  will  move  only  half 
as  fast.  This  is  why  it  takes  eight  times  as  long  to  get 
around.  The  speed  of  the  earth  in  its  orbit  is  about 
18.6  miles  per  second.  But  that  of  Neptune  is  only 
about  3.5  miles  per  second,  although  it  has  thirty  times 
as  far  to  go.  This  is  why  it  takes  more  than  one  hun- 
dred and  sixty  years  to  complete  a  revolution. 


n 

THE  PLANET  MERCURY 

To  set  forth  what  is  known  of  the  major  planets  we 
shall  take  them  up  in  the  order  of  their  distance  from  the 
sun.  The  first  planet  reached  will  then  be  Mercury.  It 
is  not  only  the  nearest  planet  to  the  sun,  but  much  the 
smallest  of  the  eight ;  so  small,  indeed,  that,  but  for  its 
situation,  it  would  hardly  be  called  a  major  planet.  Its 
diameter  is  about  two  fifths  greater  than  that  of  the 
moon,  but,  the  volumes  of  bodies  being  proportional  to 
the  cubes  of  their  diameters,  it  has  about  three  times  the 
volume  of  the  moon. 

It  has  far  the  most  eccentric  orbit  of  all  the  major 
planets,  though,  in  this  respect,  it  is  exceeded  by  some 
of  the  minor  planets  to  be  hereafter  described.  In  conse- 
quence, its  distance  from  the  sun  varies  between  wide 
limits.  At  perihelion  it  is  less  than  twenty-nine  millions 
of  miles  from  the  sun ;  at  aphelion  it  goes  out  to  a  distance 
of  more  than  forty-three  millions  of  miles.  It  performs 
its  revolution  around  the  sun  in  a  little  less  than  three 
months ;  to  speak  more  exactly,  in  eighty-eight  days.  It 
theref ore  makes  more  than  four  revolutions  in  a  year. 

Performing  more  than  four  revolutions  around  the 
sun  while  the  earth  is  performing  one,  we  readily  see 
that  it  must  pass  conjunction  with  the  sun  at  certain 
regular  though  somewhat  unequal  intervals.  To  show 


158    PLANETS  AND  THEIR  SATELLITES 

the  exact  nature  of  its  apparent  motion  let  the  inner 
circle  of  the  diagram  represent  the  orbit  of  Mercury  and 
the  outer  one  that  of  the  earth.  When  the  earth  is  at  E, 
and  Mercury  at  M,  the  latter  is  in  inferior  conjunction 
with  the  sun.  At  the  end  of  three  months  it  will  have  re- 
turned to  the  point  M,  but  it  will  not  yet  be  in  conjunc- 


FIG.  28. — Conjunctions  of  Mercury  with  the  Sun. 

tion,  because,  in  the  meantime,  the  earth  has  moved  for- 
ward in  its  orbit.  When  the  earth  reaches  a  certain  point 
F,  Mercury  will  have  reached  the  point  N  and  will  again 
be  in  inferior  conjunction.  This  revolution  from  one  in- 
ferior conjunction  to  another  is  called  the  synodic  revolu- 
tion of  the  planet.  In  the  case  of  Mercury  this  is  some- 
what less  than  one  third  more  than  the  time  of  actual 


THE  APPEARANCE  OF  MERCURY  159 

revolution ;  that  is  to  say,  the  arc  MN  is  a  little  less  than 
one  third  of  the  circle. 

Now  suppose  that  when  the  earth  is  at  E,  Mercury, 
instead  of  being  at  M  is  near  the  highest  point  A  of  the 
orbit  as  represented  in  the  figure.  It  will  then  be  at 
its  greatest  apparent  distance  from  the  sun  as  we  see 
it  from  the  earth;  or,  in  technical  language,  at  its 
greatest  east  elongation.  Being  east  of  the  sun  it  will 


/ 

..VB 


•EARTH  J-'-  / 

° - - 


Fia.  29. — Elongations  of  Mercury. 

then  set  after  the  sun,  by  a  time  generally  between 
an  hour  and  a  quarter  and  an  hour  and  a  half.  This 
is  the  most  convenient  time  for  seeing  it.  If  the  sky 
is  clear,  it  will  readily  be  seen  in  the  twilight  from  half 
an  hour  to  an  hour  after  sunset.  At  the  opposite  elonga- 
tion, near  C,  it  is  west  of  the  sun ;  then  it  rises  before  the 
sun  and  may  be  seen  in  the  morning  twilight. 

The  Surface  and  Rotation  of  Mercury 

The  best  time  to  make  a  telescopic  study  of  Mercury 
is  late  in  the  afternoon,  when  it  is  near  east  elongation, 


160    PLANETS  AND  THEIR  SATELLITES 

or  shortly  after  sunrise,  if  it  rises  before  the  sun.  Sup- 
posing it  east  of  the  sun,  it  will  probably  be  visible  in 
the  telescope  at  any  time  after  noon,  but  the  air  is  gen- 
erally disturbed  by  the  sun's  rays  so  that  it  is  hardly 
possible  to  make  a  good  observation  at  that  time.  Late 
in  the  afternoon  the  air  grows  steadier,  so  that  the  planet 
can  be  better  observed.  But,  after  sunset,  the  planet  is 
seen  through  a  continually  increasing  extent  of  atmos- 
phere, so  that  the  seeming  disturbance  again  begins  to 
increase.  Owing  to  these  circumstances  it  is  the  most 
difficult  of  all  the  planets  to  study  in  a  satisfactory  way, 
and  observers  differ  very  much  as  to  what  can  be  seen  on 
its  surface. 

The  first  observer  who  thought  he  could  see  any  fea- 
tures on  the  surface  of  this  planet  was  Schroter,  a  Ger- 
man. When  Mercury  presented  the  form  of  a  crescent 
he  fancied  that  its  south  horn  seemed  blunted  at  inter- 
vals. He  attributed  this  to  the  shadow  of  a  lofty  moun- 
tain ;  and  by  observing  the  intervals  between  the  blunted 
appearance  he  concluded  that  the  planet  revolved  on  its 
axis  in  twenty-four  hours  and  five  minutes.  But  Sir 
William  Herschel,  who  observed  at  the  same  time  with 
much  more  powerful  instruments,  could  not  see  anything 
of  the  kind. 

Until  quite  recently  nearly  all  observers  agreed  with 
Herschel  that  no  time  of  rotation  could  be  certainly  de- 
termined. But  a  few  years  since,  Schiaparelli,  observing 
with  a  fine  telescope  in  the  beautiful  sky  of  northern 
Italy,  noticed  that  the  aspect  of  the  planet  seemed  un- 
changed day  after  day.  He  was  thus  led  to  the  conclu- 


THE    PHASES    OF    MERCURY  161 

sion  that  it  always  presents  the  same  face  to  the  sun, 
as  the  moon  presents  the  same  face  to  the  earth.  This 
view  was  shared  by  Mr.  Lowell,  observing  at  the  Flag- 
staff Observatory.  But  the  observation  is  too  difficult 
to  permit  us  to  regard  the  fact  as  established.  All  that 
a  conservative  astronomer  would  be  willing  to  say  is 
that  as  yet  we  know  nothing  of  the  revolution  of  Mercury 
on  its  axis. 

Drawings  showing  the  face  of  Mercury  have  been 
made  by  several  astronomers.  As  it  is  seen  under  all 
ordinary  conditions  no  special  features  are  well  marked. 
Very  different  is  the  case  at  the  Lowell  Observatory  in 
Flagstaff,  Ariz.  The  most  singular  feature  of  its  sur- 
face in  the  latter  picture  consists  in  the  dark  lines  which 
cross  it.  These  have  not  been  seen  by  other  observers, 
and,  until  they  are  established  by  independent  evidence, 
astronomers  will  be  sceptical  as  to  their  reality.  The 
reason  of  this  will  be  stated  later  in  connection  with  the 
planet  Mars. 

Owing  to  the  various  positions  of  Mercury  relative  to 
the  sun  it  presents  phases  like  those  of  the  moon.  These 
depend  upon  the  relation  of  the  dark  and  the  illuminated 
hemispheres  relative  to  the  direction  in  which  we  see  the 
planet.  The  hemisphere  which  is  turned  away  from  the 
sun,  being  in  darkness,  is  always  invisible  to  us.  At 
superior  conjunction  the  illuminated  hemisphere  is  turned 
toward  us  and  the  planet  seems  round,  like  a  full  moon. 
As  it  moves  from  east  elongation  to  inferior  conjunction, 
more  and  more  of  the  dark  hemisphere  is  turned  toward 
us,  and  less  and  less  of  the  illuminated  one.  But  this 


162    PLANETS  AND  THEIR  SATELLITES 

disadvantage  is  counterbalanced  by  the  fact  that  the 
planet  continually  comes  nearer  during  the  interval,  so 
that  we  get  a  better  view  of  whatever  portion  of  the 
illuminated  hemisphere  may  be  visible  to  us.  Its  appar- 
ent form  and  size  at  different  times  during  its  synodic 
revolution  go  through  a  series  of  changes  similar  to  those 
shown  in  the  next  chapter  in  the  case  of  Venus. 

The  question  whether  Mercury  has  an  atmosphere  is 
also  one  on  which  opinions  differ,  the  prevailing  opinion 
being  in  the  negative.  It  seems  quite  certain  that,  if  it 
has  one,  it  is  too  rare  to  reflect  the  light  of  the  sun. 

Transits  of  Mercury 

It  will  be  readily  seen  that,  if  an  inferior  planet  re- 
volved around  the  sun  in  the  same  plane  as  the  earth,  we 
should  see  it  pass  over  the  sun's  disk  at  every  inferior 
conjunction.  But  no  two  planets  revolve  in  the  same 
plane.  Of  all  the  major  planets  the  orbit  of  Mercury 
has  the  largest  inclination  to  that  of  the  earth.  In  con- 
sequence, when  in  inferior  conjunction,  it  commonly 
passes  a  greater  or  less  distance  to  the  north  or  to  the 
south  of  the  sun.  If,  however,  it  chances  to  be  near  one 
of  its  nodes  at  the  time  in  question,  we  shall  see  it  as 
a  black  spot  passing  across  the  sun's  disk.  This  phe- 
nomenon is  called  a  transit  of  Mercury.  Such  transits 
occur  at  intervals  ranging  between  three  and  thirteen 
years.  They  are  observed  with  much  interest  by  as- 
tronomers because  it  is  possible  to  determine  with  great 
precision  the  time  at  which  the  planet  enters  upon  the 
solar  disk,  and  leaves  it  again.  Knowing  these  times, 


TRANSITS    OF    MERCURY  163 

valuable  information  is  afforded  respecting  the  exact  law 
of  motion  of  the  planet. 

The  first  observation  of  a  transit  of  Mercury  was  made 
by  Gassendi  on  November  7,  1631.  His  observation  is 
not,  however,  of  any  scientific  value  at  the  present  time, 
owing  to  the  imperfection  of  his  instruments.  A  some- 
what better  but  not  good  observation  was  made  by  Hal- 
ley,  of  England,  in  1677,  during  a  visit  to  the  island  of 
St.  Helena0  Since  that  time  the  transits  have  been  ob- 
served with  a  fair  degree  of  regularity.  The  following 
table  shows  the  transits  that  will  be  visible  during  the 
next  fifty  years,  with  the  regions  of  the  earth  in  which 
each  may  be  seen : 

1907,  November  14,  visible  in  Europe  and  eastern  United 

States. 

1914,  November  7,  visible  in  the  same  regions. 
1924,  May  7,  the  beginning  will  be  visible  on  the  Pacific 

coast,  but  the  whole  transit  only  on  the  Pacific 

Ocean  and  in  eastern  Asia. 

1927,  November  9,  visible  in  Asia  and  eastern  Europe. 
1937,  May  11,  Mercury  will  graze  the  south  limb  of 

the  sun.    The  phenomenon  will  be  visible  in  Europe, 

but  will  occur  before  the  sun  rises  in  America. 
1940,  November  10,  visible  in  the  Western  and  Pacific 

States. 
1953,  November    14,    visible    throughout    the    United 

States. 

Observations  of  transits  of  Mercury  since  1677  have 
brought  out  cne  of  the  most  perplexing  facts  of  astron- 


164    PLANETS  AND  THEIR  SATELLITES 

omy.  The  orbit  of  this  planet  is  found  to  be  slowly 
changing  its  position,  its  perihelion  moving  forward  by 
about  forty-three  seconds  per  century  farther  than  it 
ought  to  move  in  consequence  of  the  attraction  of  all  the 
known  planets.  This  deviation  was  discovered  in  1845 
by  Le  Verrier,  celebrated  as  having  computed  the  posi- 
tion of  Neptune  before  it  had  ever  been  recognised  in 
the  telescope.  He  attributed  it  to  the  attraction  of  a 
planet,  or  group  of  planets,  between  Mercury  and  the 
sun.  His  announcement  set  people  to  looking  for  the 
supposed  planet.  About  1860,  a  Dr.  Lescarbault,  a 
country  physician  of  France,  who  possessed  a  small  tele- 
scope, thought  he  had  seen  this  planet  passing  over  the 
disk  of  the  sun.  But  it  was  soon  proved  that  he  must 
have  been  mistaken.  Another  more  experienced  astron- 
omer, who  was  looking  at  the  sun  on  the  same  day,  failed 
to  see  anything  except  an  ordinary  spot.  It  was  prob- 
ably this  which  misled  the  physician-astronomer.  Now, 
for  forty  years,  the  sun  has  been  carefully  scrutinised 
and  photographed  from  day  to  day  at  several  stations 
without  anything  of  the  sort  being  seen. 

Still,  it  is  possible  that  little  planets  so  minute  as  to 
escape  detection  in  passing  over  the  sun's  disk  may  re- 
volve in  the  region  in  question.  If  so,  their  light  would 
be  completely  obscured  by  that  of  the  sky,  so  that  they 
might  not  ordinarily  be  visible.  But  there  is  still  a 
chance  that,  during  a  total  eclipse  of  the  sun,  when  the 
light  is  cut  off  from  the  sky,  they  could  be  seen.  Ob- 
servers have,  from  time  to  time,  looked  for  them  during 
total  eclipses.  In  one  instance  something  of  the  sort  was. 


INTRAMERCURIAL    PLANETS  165 

supposed  to  be  found.  During  the  eclipse  of  1878, 
Professor  Watson,  of  Ann  Arbor,  and  Professor  Lewis 
Swift,  both  able  and  experienced  observers,  thought  that 
they  had  detected  some  such  bodies.  But  critical  exam- 
ination left  no  doubt  that  what  Watson  saw  was  a  pair 
of  fixed  stars  which  had  always  been  in  that  place.  How 
it  was  with  the  observations  of  Professor  Swift  has  never 
been  certainly  ascertained,  because  he  was  not  able  to  lay 
down  the  position  with  such  certainty  that  positive  con- 
clusions could  be  drawn. 

Notwithstanding  such  failures,  observers  have  repeated 
the  search  during  several  of  the  principal  total  eclipses. 
The  writer  did  so  during  the  eclipse  of  1869,  and  again 
during  that  of  1878,  the  search  being  made  with  a  small 
telescope.  In  recent  times  the  powerful  agency  of 
photography  has  been  invoked  by  Professors  Pickering 
and  Campbell  during  the  eclipses  of  1900  and  1901. 
Campbell's  results  during  the  latter  eclipse  were  the  most 
decisive  yet  reached.  With  his  photographic  telescope 
some  fifty  stars  were  photographed,  some  as  faint  as  the 
eighth  magnitude,  but  they  were  all  found  to  be  known 
objects.  It  therefore  seems  certain  that  there  can  be  no 
intramercurial  much  brighter  than  the  eighth  magnitude. 
It  would  take  hundreds  of  thousands  of  such  planets  as 
this  to  produce  the  observed  motion  of  Mercury.  So  great 
a  number  of  these  bodies  would  produce  a  far  brighter 
illumination  of  the  sky  than  any  that  we  see.  The  result 
therefore  seems  to  be  conclusive  against  the  view  that  the 
motion  of  the  perihelion  of  Mercury  can  be  produced  by 
intramercurial  planets.  In  addition  to  all  these  difficul- 


166    PLANETS  AND  THEIR  SATELLITES 

tics  in  supposing  the  planet  to  exist  we  have  the  difficulty 
that,  if  it  did  exist,  it  would  produce  a  similar  though 
smaller  change  in  the  position  of  the  nodes  of  either  Mer- 
cury or  Venus,  or  both. 

Altogether,  the  evidence  seems  conclusive  against  the 
reality  of  any  bodies  whose  attraction  could  produce  the 
observed  deviation,  which  still  remains  unexplained.  The 
most  recent  supposition  on  the  subject  is  that  the  force 
of  gravitation  deviates  slightly  from  the  law  of  the  in- 
verse square.  But  this  requires  farther  investigation. 


Ill 

THE  PLANET  VENUS 

OF  all  the  star-like  objects  in  the  heavens  the  planet 
Venus  is  the  most  brilliant.  The  sun  and  moon  are  the 
only  heavenly  bodies  outshining  it.  In  a  clear  and  moon- 
less evening  it  may  be  seen  to  cast  a  shadow.  If  an 
observer  knows  exactly  where  to  look  for  it,  and  has  a 
well-focused  eye,  it  can  be  seen  in  the  daytime  when  near 
the  meridian,  provided  that  the  sun  is  not  in  its  immediate 
neighbourhood.  When  it  is  east  of  the  sun  it  may  be  seen 
in  the  west,  faintly  before  sunset  and  growing  continually 
brighter  as  the  light  diminishes.  When  west  of  the  sun 
it  rises  in  the  morning  before  the  sun,  and  may  then  be 
seen  in  the  east.  Under  these  circumstances  it  has  been 
called  the  evening  and  morning  star  respectively.  The 
ancients  called  it  Hesperus  when  an  evening  star,  and 
Phosphorus  when  a  morning  star.  It  is  said  that,  in  the 
early  history  of  our  race,  Hesperus  and  Phosphorus  were 
not  known  to  be  the  same  body. 

If  Venus  is  examined  with  the  telescope,  even  one  of 
low  power,  it  will  be  seen  to  exhibit  phases  like  those  of 
the  moon.  This  fact  was  ascertained  by  Galileo  when 
he  first  directed  his  telescope  toward  the  planet,  and  af- 
forded him  strong  evidence  of  the  truth  of  the  Coperni- 
can  System.  In  accordance  with  a  custom  of  the  time  he 
published  this  discovery  in  the  form  of  an  anagram — a 


168    PLANETS  AND  THEIR  SATELLITES 

collection  of  letters  which,  when  subsequently  put  to- 
gether would  state  the  discovery.  Translated  into  Eng- 
lish the  anagram  read,  "The  mother  of  the  loves  emulates 
the  phases  of  Cynthia." 

What  we  have  said  of  the  synodic  motion  of  Mercury 
applies  in  principle  to  Venus,  and  need  not  therefore  be 
repeated.  In  the  following  cut  the  apparent  size  of  the 
planet  is  shown  in  various  parts  of  its  synodic  orbit.  As 
the  planet  passes  from  superior  to  inferior  conjunction 
its  globe  continually  grows  larger  in  apparent  size, 


FIG.  30. — Phases  of  Venus  in  Different  Points  of  its  Orbit. 

though  we  cannot  see  its  entire  outline.  But  the  fraction 
of  the  disk  illuminated  continually  becomes  smaller,  first 
having  the  shape  of  a  half  moon,  and  then  the  shape  of 
a  crescent,  which  grows  thinner  and  thinner  up  to  the 
time  of  inferior  conjunction.  In  the  latter  position  the 
dark  hemisphere  is  turned  toward  us  and  the  planet  is 
invisible.  Venus  is  at  its  greatest  brightness  about  half- 
way between  inferior  conjunction  and  greatest  elonga- 
tion. It  then  sets  about  two  hours  after  the  sun,  if  east 
of  it,  and  rises  about  two  hours  before  the  sun,  if  west 
of  it. 


ROTATION    OF   VENUS  •     169 

Rotation  of  Venus 

The  question  of  the  rotation  of  Venus  has  interested 
astronomers  and  the  public  ever  since  the  time  of  Galileo. 
But  the  difficulty  of  learning  anything  certain  on  the 
subject  is  very  great,  owing  to  the  peculiar  glare  of  the 
planet.  When  seen  through  a  telescope  no  sharp  and 
well-defined  markings  are  visible.  Instead  of  this  there 
is  a  glare  on  the  surface,  varying  by  gentle  gradations 
from  one  region  to  another,  as  if  we  were  looking  upon 
a  globe  of  polished  but  slightly  tarnished  metal.  Never- 
theless, various  observers  have  supposed  that  they  could 
distinguish  bright  or  dark  spots.  As  far  back  as  1667 
Cassini  concluded  from  these  seeming  spots  that  the 
planet  revolved  on  its  axis  in  a  little  less  than  twenty- 
four  hours.  During  the  next  century  Blanchini,  an 
Italian  observer,  published  an  extensive  treatise  on  the 
subject,  illustrated  with  many  drawings  of  the  planet. 
His  conclusion  was  that  Venus  required  more  than  twenty- 
four  days  to  revolve  on  its  axis.  Cassini,  the  son,  de- 
fended his  father's  conclusion  by  claiming  that  the  planet 
Jiad  always  made  one  revolution  and  a  little  more  between 
the  times  of  Blanchini's  observations  on  successive  even- 
ings. Thus  the  Italian  astronomer  would  naturally  see 
the  spots  on  successive  evenings  a  little  farther  advanced, 
and  estimated  the  motion  by  this  advance,  not  being  aware 
that  a  whole  revolution  had  been  made  during  the  interval. 
At  the  end  of  twenty-four  days  the  same  hemisphere  of 
the  planet  would  be  presented  to  the  earth  as  before,  the 
number  of  revolutions  in  the  meantime  being  twenty-five. 


170    PLANETS  AND  THEIR  SATELLITES 

Schroter  tried  to  decide  the  question  for  Venus  in  the 
same  way  that  he  supposed  himself  to  have  decided  it  for 
Mercury.  He  directed  his  attention  especially  to  the  fine 
sharp  horns  of  the  crescent,  when  the  planet  was  nearly 
between  the  earth  and  the  sun.  At  certain  intervals  he 
supposed  one  of  them  to  be  a  little  blunted.  Ascribing 
this  appearance  to  the  shadow  of  a  high  mountain,  he 
concluded  that  the  time  of  rotation  was  twenty-three 
hours  twenty-one  minutes. 

From  the  time  of  Schroter  no  one  professed  to  throw 
any  more  light  on  the  question  until  1832.  Then  De 
Vico,  of  Rome,  announced  4hat  he  had  rediscovered  the 
markings  found  by  Blanchini.  He  concluded  that  the 
planet  rotated  in  twenty-three  hours  twenty -one  minutes, 
in  agreement  with  Schroter's  result. 

This  close  agreement  between  the  results  of  observa- 
tions by  four  distinguished  observers  led  to  the  very  gen- 
eral acceptance  of  twenty-three  hours  twenty-one  minutes 
as  the  time  of  rotation  of  the  planet.  But  there  was  much 
to  be  said  on  the  other  side.  The  great  Herschel,  with 
the  most  powerful  telescopes  that  had  ever  been  made,  was 
never  able  to  make  out  any  permanent  markings  on  Venus. 
If  anything  like  a  spot  appeared,  it  varied  and  disap- 
peared again  so  rapidly  that  no  evidence  of  rotation  could 
be  afforded  by  it.  This  negative  result  has  always  been 
reached  by  the  large  maj  ority  of  observers. 

But  a  new  and  surprising  theory  has  been  recently  put 
forth  by  Schiaparelli,  and  maintained  by  Lowell.  This  is 
that  Venus  rotates  on  its  axis  in  the  same  period  that  it 
revolves  around  the  sun;  in  other  words  both  Mercury 


ROTATION    OF   VENUS  171 

and  Venus  always  present  the  same  face  to  the  sun,  as  the 
moon  presents  the  same  face  to  the  earth.  Schiaparelli 
reached  this  conclusion  by  noticing  that  a  number  of  ex- 
ceedingly faint  spots  could  be  seen  on  the  southern  hemi- 
sphere of  Venus  for  several  days  in  succession  in  the 
same  position  day  after  day.  He  could  observe  the  planet 
through  several  hours  on  each  day,  and  the  constancy 
of  the  spots  precluded  the  idea  that  the  planet  made  one 
rotation  and  a  little  more  in  the  course  of  a  day.  Lowell 
was  led  to  the  same  conclusion  by  careful  study  of  the 
planet  at  his  Arizona  observatory. 

The  latest  conclusion  has  been  reached  by  the  spectro- 
scope. We  have  already  explained  how,  with  this  instru- 
ment, it  can  be  determined  whether  a  heavenly  body  is 
moving  toward  us  or  from  us.  The  principle  applies 
to  a  planet  which  we  see  by  the  reflected  light  of  the  sun 
as  well  as  to  a  star.  Hence,  if  Venus  rotates,  one  part 
of  its  disk  will  be  moving  toward  us,  and  the  other  from 
us.  By  comparing  the  dark  lines  of  the  spectrum  shown 
by  the  two  edges  of  the  disk  of  Venus  it  can  then  be  de- 
termined how  various  points  of  the  disk  are  moving  with 
respect  to  the  earth.  It  was  thus  found  by  Belopolsky 
that  the  planet  was  affected  by  a  quite  rapid  rotation. 
The  observation  is  so  difficult,  and  the  displacement  of 
the  lines  so  small,  that  it  was  not  possible  to  state  a  very 
certain  result,  although  the  general  fact  was  made  very 
probable.  On  the  whole  we  must  regard  this  conclusion 
as  the  most  likely  that  has  yet  been  reached,  although  it 
is  at  variance  with  the  observations  of  Schiaparelli,  as 
well  as  those  of  the  Lowell  Observatory.  But  the  spectro- 


172    PLANETS  AND  THEIR  SATELLITES 


scopic  observations  have  not  yet  been  made  with  sufficient 
precision  to  teach  us  the  exact  time  of  revolution.  Re- 
cent discoveries  as  to  the  nature  of  the  atmosphere  of 
Venus  make  it  almost  certain  that  all  the  observers  who 
supposed  that  they  saw  markings  on  the  planet  were 
mistaken. 

Atmosphere  of  Venus 

It  is  now  well  established  that  Venus  is  surrounded 
by  an  atmosphere  which  is  probably  denser  than  that  of 
the  earth.  This  was  shown  in  a  remarkable  and  interest- 


PART  OF  THE  SUN. 


FIG.  SI.— Effect  of  the  Atmosphere  of  Venus  during  tJie  Transit  oj  1882. 

ing  way  during  the  transit  of  Venus  over  the  sun's  disk 
in  1882,  which  was  observed  by  the  writer  at  the  Cape 
of  Good  Hope.  When  the  planet  was  a  little  more  than 
halfway  on  the  disk,  its  outer  edge  appeared  illuminated, 
as  shown  on  the  figure.  This  illumination,  however,  did 
not  commence  at  the  middle  point  of  the  arc,  as  it 


ATMOSPHERE    OF    VENUS  173 

should  have  done  had  it  been  caused  by  regular  refrac- 
tion, but  commenced  at  a  point  quite  near  one  end  of  the 
arc.  This  appearance  was  explained  by  Russell,  of 
Princeton,  who  showed  that  the  atmosphere  is  so  full 
of  vapour  that  we  cannot  see  the  light  of  the  sun  by 
direct  refraction  through  it.  What  we  see  is  an  illu- 
minated stratum  of  clouds  or  vapour  floating  in  an  at- 
mosphere. Such  being  the  case,  it  is  not  at  all  likely  that 
astronomers  on  the  earth  can  ever  see  the  solid  body  of 
the  planet  through  these  clouds.  Hence  the  supposed 
spots  could  only  have  been  temporary  clouds,  continually 
changing. 

To  illustrate  the  illusions  to  which  the  sight  of  even 
good  observers  may  be  subject,  we  may  mention  the  fact 
that  several  such  observers  have  supposed  the  whole  hemi- 
sphere of  Venus  to  be  visible  when  the  planet  was  near 
inferior  conjunction.  It  then  had  the  appearance  fa- 
miliarly known  as  "the  new  moon  in  the  old  moon's 
arms,"  with  which  everyone  who  observes  our  satellite 
when  a  narrow  crescent  is  familiar.  In  the  case  of  the 
moon  it  is  well  known  that  we  thus  see  the  dark  hemi- 
^sphere  by  the  light  reflected  from  the  earth.  But  in 
the  case  of  Venus  there  is  no  possibility  of  a  sufficient 
reflection  of  light  from  the  earth,  or  any  other  body. 
The  appearance  has  sometimes  been  explained  by  a  possi- 
ble phosphorescence  covering  the  whole  hemisphere  of 
Venus.  But  it  is  more  likely  due  to  an  optical  illusion. 
It  has  generally  been  seen  in  the  daytime,  when  the 
sky  is  brightly  illuminated,  and  when  any  faint  light 
like  that  of  phosphorescence  would  be  completely  in- 


PLANETS  AND  THEIR  SATELLITES 

visible.  To  whatever  we  might  attribute  the  light,  it 
ought  to  be  seen  far  better  after  the  end  of  twilight  in  the 
evening  than  during  the  daytime.  The  fact  that  it  is  not 
seen  then  seems  to  be  conclusive  against  its  reality. 

The  appearance  illustrates  a  well-known  psychological 
law,  that  the  imagination  is  apt  to  put  in  what  it  is  ac- 
customed to  see,  even  when  the  object  is  not  there.  We 
are  so  accustomed  to  the  appearance  on  the  moon  that 
when  we  look  at  Venus  the  similarity  of  the  general  phe- 
nomena leads  us  to  make  this  supposed  familiar  addition 
to  it. 

Has  Venus  a  Satellite? 

During  the  past  two  centuries  several  observers  have 
from  time  to  time  thought  that  they  saw  a  satellite  of 
Venus.  Countless  observers,  with  good  telescopes,  have 
Seen  nothing  of  the  sort.  We  may  safely  say  that  Venus 
has  no  satellite  visible  in  the  most  powerful  telescopes 
of  our  time.  Quite  likely  these  supposed  satellites  were 
geeming  objects  quite  familiar  to  astronomers  under  the 
name  of  "ghosts."  These  are  sometimes  seen  when  a 
telescope  is  pointed  at  a  bright  object,  and  are  due  to  a 
double  reflection  of  light  in  the  lenses  either  of  the  ob j  ect- 
glass  or  the  eyepiece. 

A  few  years  ago  the  writer  received  a  letter  from  the 
owner  of  a  very  large  telescope  in  England  stating  that, 
by  great  care,  he  could  see  a  very  faint,  round,  and  well- 
defined  aureole  of  light  around  the  planet  Mars.  He 
desired  to  know  whether  the  object  could  be  real,  or  how 
the  appearance  was  to  be  explained.  In  reply,  he  was 
informed  that  such  an  appearance  would  be  produced 


TRANSITS    OF    VENUS  175 

by  the  double  reflection  of  light  between  the  two  inner 
lenses  of  the  object-glass,  provided  their  curvatures  were 
nearly,  but  not  exactly  the  same.  It  was  suggested  that 
he  point  the  telescope  at  Sirius  and  see  if  a  similar  ap- 
pearance did  not  surround  the  star.  He  probably  found 
that  such  was  the  case. 

Transits  of  Venus 

The  transits  of  Venus  across  the  sun's  disk  are  among 
the  rarest  phenomena  of  astronomy,  as  they  occur,  on 
the  average,  only  once  in  sixty  years.  For  many  cen- 
turies past  and  to  come  there  will  be  a  regular  cycle, 
bringing  about  four  transits  in  two  hundred  and  forty- 
three  years.  The  intervals  between  the  transits  are  one 
hundred  and  five  and  a  half  years,  eight  years,  one  hun- 
dred and  twenty-one  and  a  half  years,  eight  years ;  then 
one  hundred  and  five  and  a  half  years  again,  and  so  on. 
The  dates  of  the  last  six  transits  and  the  two  next  to 
come  are  as  follows: 

1631,  December  7,  1874,  December  9, 

1639,  December  4,  1882,  December  6, 

1761,  June  5,  2004,  June  8, 

1769,  June  3,  2012,  June  6. 

It  will  be  seen  that  no  person  now  living  is  likely  to  see 
this  phenomenon,  as  the  next  transit  does  not  occur  until 
2004.  Yet,  the  time  when  Venus  will  appear  upon  the 
disk  on  June  8  of  that  year  can  now  be  predicted  for  any 
point  on  the  earth's  surface,  within  a  minute  or  two. 


176    PLANETS  AND  THEIR  SATELLITES 

The  interest  which  has  attached  to  these  transits  dur- 
ing the  past  century  arose  from  the  fact  that  they  were 
supposed  to  afford  the  best  method  of  determining  the 
distance  of  the  sun  from  the  earth.  This  fact  and  the 
rarity  of  the  phenomenon  led  to  the  last  four  transits 
being  observed  on  a  large  scale.  In  1761,  and  again  in 
1769,  the  leading  maritime  nations  sent  observers  to 
various  parts  of  the  world  to  note  the  exact  time  at 
which  the  planet  entered  upon  and  left  the  sun's  disk.  In 
1874  and  1882,  expeditions  were  fitted  up  on  a  large 
scale  by  the  United  States,  Great  Britain,  France,  and 
Germany.  On  the  first  of  these  occasions  American  par- 
ties occupied  stations  in  China,  Japan,  and  eastern 
Siberia  on  the  north,  and  in  Australia,  New  Zealand, 
Chatham  Island,  and  Kerguelen  Island  in  the  south.  In 
1882  it  was  not  necessary  to  send  out  so  many  expedi- 
tions, because  the  transit  was  visible  in  this  country.  In 
the  southern  hemisphere  stations  were  occupied  at  the 
Cape  of  Good  Hope  and  other  points.  The  observations 
made  by  these  expeditions  proved  of  great  value  in  de- 
termining the  future  motions  of  Venus,  but  it  was  found 
that  other  methods  of  determining  the  distance  of  the 
sun  would  lead  to  a  more  certain  result. 


rv 

THE  PLANET  MAKS 

MORE  public  interest  has  in  recent  years  been  con- 
centrated on  the  planet  Mars  than  on  any  other.  Its 
resemblance  to  our  earth,  its  supposed  canals,  oceanss 
climate,  snowfall,  etc.,  have  all  tended  to  interest  us  in 
its  possible  inhabitants.  At  the  risk  of  disappointing 
those  readers  who  would  like  to  see  certain  proof  that  our 
neighbouring  world  is  peopled  with  rational  beings,  I 
shall  endeavour  to  set  forth  what  is  actually  known  on 
the  subject,  distinguishing  it  from  the  great  mass  of  illu- 
sion and  baseless  speculation  which  has  crept  into  popular 
journals  during  the  past  twenty  years. 

We  begin  with  some  particulars  which  will  be  useful 
in  recognising  the  planet.  Its  period  of  revolution  is 
six  hundred  and  eighty-seven  days,  or  forty-three  days 
less  than  two  years.  If  the  period  were  exactly  two  years, 
it  would  make  one  revolution  while  the  earth  made  two, 
"and  we  should  see  the  planet  in  opposition  at  regular  in- 
tervals of  two  years.  But,  as  it  moves  a  little  faster  than 
this,  it  takes  the  earth  from  one  to  two  months  to  catch 
up  with  it,  so  that  the  oppositions  occur  at  intervals  of 
two  years  and  one  or  two  months.  This  excess  of  one  or 
two  months  makes  up  a  whole  year  after  eight  opposi- 
tions ;  consequently,  at  the  end  of  about  seventeen  years, 
Mars  will  again  be  in  opposition  at  the  same  time  of  the 


178    PLANETS  AND  THEIR  SATELLITES 

year,  and  near  the  same  point  of  its  orbit,  as  before.  In 
this  period  the  earth  will  have  made  seventeen  revolutions 
and  Mars  nine. 

The  difference  of  a  month  or  so  in  the  interval  be- 
tween  oppositions  is  due  to  the  great  eccentricity  of  the 
orbit,  which  is  larger  than  that  of  any  other  major 
planet  except  Mercury.  Its  value  is  0.093,  or  nearly 
one  tenth.  Hence,  when  in  perihelion,  it  is  nearly  one 
tenth  nearer  the  sun  than  its  mean  distance,  and  when 
in  aphelion  nearly  one  tenth  farther.  Its  distance  from 
the  earth  at  opposition  will  be  different  by  the  same 
amount,  measured  in  miles,  and  hence  in  a  much  larger 
proportion  to  the  distance  itself.  If  opposition  occurs 
when  the  planet  is  near  perihelion,  the  distance  from 
earth  is  about  forty-three  million  miles;  but  if  near 
the  aphelion,  about  sixty  million  miles.  The  result  of 
this  is  that,  at  a  perihelion  opposition,  which  can  occur 
only  in  September,  the  planet  will  appear  more  than 
three  times  as  bright  as  at  an  aphelion  opposition,  occur- 
ing  in  February  or  March.  An  opposition  occurred 
near  the  end  of  March,  1903 ;  the  next  following  early 
in  May,  1905.  We  shall  then  have  oppositions  near  the 
end  of  June,  1907,  and  in  August,  1909,  which  will  be 
quite  near  to  perihelion . 

Mars,  when  near  opposition,  is  easily  recognised  by  its 
brilliancy,  and  by  the  reddish  colour  of  its  light,  which  is 
very  different  from  that  of  most  of  the  stars.  It  is 
curious  that  a  telescopic  view  of  the  planet  does  not  give 
so  strong  an  impression  of  red  light  as  does  the  naked  eye 
view. 


SURFACE  AND  ROTATION  OF  MARS  179 

The  Surface  and  Rotation  of  Mars 

The  great  Huygens,  who  flourished  between  1650  and 
1700,  studying  Mars  with  the  telescope,  was  the  first  one 
to  recognise  the  variegated  character  of  its  surface,  and 
to  make  a  drawing  of  the  appearance  which  it  presented. 
The  features  delineated  by  Huygens  can  be  recognised 
arid  identified  to  this  day.  By  watching  them  it  was  easy 
to  see  that  the  planet  rotated  on  its  axis  in  a  little  more 
than  one  of  our  days  (£4h.  37m.). 

This  time  of  rotation  is  the  only  definite  and  certain 
one  among  all  the  planets  besides  the  earth.  For  two 
hundred  years  Mars  has  rotated  at  exactly  this  rate,  and 
there  is  no  reason  to  suppose  that  the  time  will  change 
appreciably  any  more  than  the  length  of  our  day  will. 
The  close  approach  to  one  of  our  days,  the  excess  being 
only  thirty-seven  minutes,  leads  to  the  result  that,  on 
successive  nights,  Mars  will,  at  the  same  hour,  present 
nearly  the  same  face  to  the  earth.  But,  owing  to  the  ex- 
cess in  question,  it  will  always  be  a  little  farther  behind 
on  any  one  night  than  on  the  night  before,  so  that,  at  the 
-end  of  forty  days,  we  shall  have  seen  every  part  of  the 
planet  that  is  presented  to  the  earth. 

All  that  was  known  of  Mars  up  to  a  quite  recent 
period  could  be  embodied  in  a  map  of  the  planet,  showing 
the  bright  and  dark  regions  of  its  surface,  and  in  the 
fact  that  a  white  cap  would  be  generally  seen  to  surround 
each  of  its  poles.  When  a  pole  was  inclined  toward  us, 
and  therefore  toward  the  sun,  this  cap  gradually  grew 
smaller,  enlarging  again  when  the  pole  was  turned  from 


180    PLANETS  AND  THEIR  SATELLITES 

the  sun.  In  the  latter  case  it  would  be  invisible  from  the 
earth,  so  that  the  growth  would  be  recognised  only  by 
its  larger  size  when  it  again  came  into  sight.  These  caps 
were  naturally  supposed  to  be  snow  and  ice  which  formed 
around  the  poles  during  the  Martian  winter,  and  partly 
or  wholly  melted  away  during  the  summer. 

The  Canals  of  Mars 

In  1877  commenced  Schiaparelli's  celebrated  observa- 
tions on  the  surface  of  Mars,  and  his  announcement  of 
the  so-called  canals.  The  latter  consisted  of  streaks 
passing  from  point  to  point  on  the  planet,  and  slightly 
darker  than  the  general  surface.  Seldom  has  more  mis- 
apprehension been  caused  by  a  mistranslation  than  in 
the  present  case.  Schiaparelli  called  these  streaks  canale, 
an  Italian  word  meaning  channels.  He  called  them  so 
because  it  was  then  supposed  that  the  darker  regions 
of  the  surface  were  oceans,  and  the  streams  connecting 
the  oceans  were  therefore  supposed  to  be  water,  and  so 
were  called  channels.  But  the  translation  "canals"  led 
to  a  widespread  notion  that  these  streaks  were  the  works 
of  inhabitants,  as  canals  on  the  earth  are  the  works  of 
men. 

Up  to  the  present  time  there  is  some  disagreement  be- 
tween observers  and  astronomical  authorities  on  the  sub- 
ject of  these  channels.  This  arises  from  the  fact  that 
they  are  not  well-defined  features  on  an  otherwise  uni- 
form surface.  Everywhere  on  the  planet  are  found 
variations  of  shade — light  and  dark  patches,  so  faint 
and  ill  defined  that  it  is  generally  difficult  to  assign  exact 


f 

CO 


182    PLANETS  AND  THEIR  SATELLITES 

form  and  outline  to  them,  running  into  each  other  by 
insensible  gradations.  The  extreme  difficulty  of  making 
them  out  at  all,  and  the  variety  of  aspects  they  present 
under  different  illuminations  and  in  different  states  of 
our  atmosphere,  has  resulted  in  a  great  variety  of  in- 
consistent delineations  of  these  objects.  At  one  extreme 
we  have  the  drawings  made  by  the  observers  at  the  Lowell 
Observatory  at  Flagstaff,  Ariz.  These  show  the  chan- 
nels as  fine  dark  lines,  so  numerous  as  to  form  a  network 
covering  the  greater  part  of  the  surface  of  the  planet. 
In  Schiaparelli's  map  they  are  rather  broad  faint  bands, 
not  nearly  so  well  defined  as  in  Lowell's  drawings.  Low- 
ell's channels  are  much  more  numerous  than  those  seen 
by  Schiaparelli.  We  might  therefore  suppose  that  all 
marked  by  the  latter  could  be  identified  on  Lowell's  map. 
But  such  is  far  from  being  the  case ;  there  is  only  a  gen- 
eral resemblance  between  the  features  seen  at  the  two 
stations.  One  of  the  most  curious  features  of  Lowell's 
drawings  is  that  the  points  where  the  channels  cross  each 
other  are  marked  by  dark  round  spots  like  circular 
lakes.  No  such  spots  as  these  are  shown  on  Schiaparelli's 
map. 

One  of  the  best  marked  features  of  Mars  is  a  large, 
dark,  nearly  circular  spot,  surrounded  by  white,  which 
is  called  Lacus  Soils,  or  the  Lake  of  the  Sun.  All  ob- 
servers agree  on  this.  They  also  agree  in  a  considerable 
part  as  to  certain  faint  streaks  or  channels  extending 
from  this  lake.  But  when  we  go  farther  we  find  that 
they  do  not  agree  as  to  the  number  of  these  channels, 
nor  is  there  an  exact  agreement  as  to  the  surrounding 


THE    CANALS    OF    MARS  183 

features.  It  will  be  interesting  to  study  two  drawings 
of  this  region  made  at  the  Lick  Observatory,  probably 
under  the  best  possible  conditions,  by  Campbell  and 
Hussey,  respectively. 

It  is  not  likely  that  any  observatory  is  more  favoured 
by  its  atmosphere  for  observations  on  this  planet  than 
the  Lick  on  Mount  Hamilton.  Its  telescope  is  the  largest 
and  finest  in  the  world  that  has  ever  been  especially 


FIGS.  33-34. — Dr arcings  of  Lacus  Soils  on  Mars,  by  Messrs.  Campbell  and 

H-ussey. 

"directed  to  Mars,  and  Barnard  is  one  of  the  most  cautious 
observers.  It  is  therefore  very  noteworthy  that  on  the 
face  of  Mars,  as  presented  to  Barnard  in  the  Lick  tele- 
scope, the  features  do  not  quite  correspond  to  the  chan- 
nels of  Schiaparelli  and  Lowell.  When  the  air  was  ex- 
ceptionally steady  he  could  see  a  vast  number  of  minute 
and  very  faint  markings,  which  were  not  visible  in  the 
smaller  telescopes  used  by  the  other  observers.  These 


184    PLANETS  AND  THEIR  SATELLITES 

were  so  intricate  that  it  was  impossible  to  represent  them 
on  a  drawing.  They  were  not  confined  to  the  brighter 
region^  of  the  planet,  or  the  supposed  continents,  but 
were  found  to  be  more  numerous  on  the  so-called  seas. 
They  showed-  no  such  regularity  that  they  could  be  con- 
sidered as  channels  running  from  one  region  to  another. 
The  eye  could  indeed  trace  darker  streaks  here  and  there, 
and  some  of  these  corresponded  to  the  supposed  channels, 
but  they  were  far  more  irregular  than  the  features  on 
Schiaparelli's  and  Lowell's  maps. 

The  matter  was  explained  by  Cerulli,  a  careful  and  in- 
dustrious Italian  observer,  in  a  way  which  seems  very 
plausible.  He  found  that  after  he  had  been  studying 
Mars  for  two  years  he  was  able,  by  looking  at  the  moon 
through  an  opera  glass,  to  see,  or  fancy  he  saw,  lines 
and  markings  upon  its  surface  similar  to  those  of  Mars. 
This  phenomenon  is  not  to  be  regarded  as  a  pure  illusion 
On  the  one  hand,  or  an  exact  representation  of  objects 
on  the  other.  Tt  grows  out  of  the  spontaneous  action  of 
the  eye  in  shaping  slight  and  irregular  combinations  of 
light  and  shade,  too  minute  to  be  separately  made  out, 
into  regular  forms. 

Probable  Nature  of  the  Channels 

The  probable  facts  of  the  case  may  be  summed  up  as 
follows : 

1.  The  surface  of  Mars  is  extremely  variegated  by 
regions  differing  in  shade,  and  having  no  very  distinct 
outlines. 

2.  There  are  numerous  dark  streaks,  generally  some- 


THE    ATMOSPHERE    OF    MARS         185 

what  indefinite  in  outline,  extending  through  consider- 
able distances  across  the  planet. 

3.  In  many  cases  the  dark  portions  appear  as  if 
chained  together  to  a  greater  or  less  extent,  and  thus 
give  rise  to  the  appearance  of  long  dark  channels. 

The  appearance  on  which  this  third  phenomenon, 
which  we  may  regard  as  identical  with  that  observed  by 
Cerulli,  is  based,  may  be  well  illustrated  by  looking,  with 
a  magnifying  glass,  at  a  stippled  portrait  engraved  on 
steel.  Nothing  will  then  be  seen  but  dots,  arranged  in 
various  lines  and  curves.  But  take  away  the  magnifying 
glass  and  the  eye  connects  these  dots  into  a  well-defined 
collection  of  features  representing  the  outlines  of  the 
human  face.  As  the  eye  makes  an  assemblage  of  dots  into 
a  face,  so  may  it  make  the  minute  markings  on  the  planet 
Mars  into  the  form  of  long,  unbroken  channels. 

The  features  which  we  have  hitherto  described  do  not 
belong  to  the  two  polar  regions  of  the  planet.  Even  wrhen 
the  snowcaps  have  melted  away,  these  regions  are  seen 
so  obliquely  that  it  would  be  difficult  to  trace  any  well- 
defined  features  upon  them.  The  interesting  question  is 
whether  the  caps  which  cover  them  are  really  snow  which 
falls  during  the  Martian  winter  and  melts  again  when 
the  sun  once  more  shines  on  the  polar  regions.  To  throw 
light  on  this  question  we  have  to  consider  some  recent 
results  as  to  the  atmosphere  of  the  planet. 

The  Atmosphere  of  Mars 

All  recent  observers  are  agreed  that,  if  Mars  has  any 
atmosphere  at  all,  it  is  much  rarer  than  our  own,  and 


186    PLANETS  AND  THEIR  SATELLITES 

contains  little  or  no  aqueous  vapour.  This  conclusion  is 
reached  from  observations  both  with  the  telescope  and 
the  spectroscope.  The  most  careful  eye  observations  of 
the  planet  show  that  the  features  are  rarely,  if  ever,  ob- 
scured by  anything  which  can  be  considered  as  clouds  in 
the  Martian  atmosphere.  It  is  true  that  the  features  are 
not  always  seqn  with  the  same  distinctness ;  but  the  varia- 
tions in  the  appearance  are  no  greater  than  would  be  due 
to  the  changes  in  the  steadiness  and  purity  of  our  own 
atmosphere,  through  which  the  astronomer  necessarily 
makes  his  observations.  Although,  near  the  edge  of  the 
apparent  disk  of  the  planet,  the  features  appear  to  be 
softened,  as  if  seen  through  a  greater  thickness  of  the  at- 
mosphere, this  appearance  is,  at  least  in  part,  due  to  the 
obliquity  of  the  line  of  sight,  which  prevents  our  getting 
so  good  a  view  of  the  edge  of  the  disk  as  of  its  centre. 
Something  of  the  same  sort  may  be  noticed  when  the 
moon  is  viewed  with  the  naked  eye  or  an  opera  glass.  Yet 
it  is  quite  possible  that  a  certain  amount  of  the  softening 
may  be  due  to  a  rare  atmosphere  on  Mars. 

The  most  careful  spectroscopic  examination  of  the 
planet  was  made  by  Campbell,  who  compared  its  spec- 
trum with  that  of  the  moon.  He  could  not  detect  the 
slightest  difference  between  the  two  spectra.  Now,  if 
Mars  had  an  atmosphere  capable  of  exerting  a  strong 
selective  absorption  on  light,  we  should  see  lines  in  the 
spectrum  due  to  this  absorption  or,  at  least,  some  of  the 
lines  would  be  strengthened.  Our  general  conclusion 
therefore  must  be  that,  while  it  is  quite  probable  that 
Mars  has  an  atmosphere,  it  is  one  of  considerable  rarity, 


IS   THERE    SNOW   ON   MARS?          1ST 

and  docs  not  bear  much  aqueous  vapour.  Now  snow  can 
fall  only  through  the  condensation  of  aqueous  vapour  in 
the  atmosphere.  It  does  not  therefore  seem  likely  that 
much  snow  can  fall  on  the  polar  regions  of  Mars. 

Another  consideration  is  that  the  power  of  the  sun's 
rays  to  melt  snow  is  necessarily  limited  by  the  amount  of 
heat  that  they  convey.  In  the  polar  regions  of  Mars  the 
rays  fall  with  a  great  obliquity,  and  even  if  all  the  heat 
conveyed  by  them  were  absorbed,  only  a  few  feet  of  snow 
could  be  melted  in  the  course  of  the  summer.  But 
far  the  larger  proportion  of  this  heat  must  be  reflected 
from  the  white  snow,  which  is  also  kept  cool  by  the 
intense  radiation  into  perfectly  cold  space.  We  there- 
fore conclude  that  the  amount  of  snow  that  can  fall 
and  melt  around  the  polar  regions  of  Mars  must  be 
very  small,  being  probably  measured  by  inches  at  the 
outside. 

As  the  thinnest  fall  of  snow  would  suffice  to  produce  a 
white  surface,  this  does  not  prove  that  the  caps  are  not 
snow.  But  it  seems  more  likely  that  the  appearance  is 
produced  by  the  simple  condensation  of  aqueous  vapour 
.upon  the  intensely  cold  surface,  producing  an  appear- 
ance similar  to  that  of  hoarfrost,  which  is  only  frozen 
dew.  This  seems  to  me  the  most  plausible  explanation 
of  the  polar  caps,,  It  has  also  been  suggested  that  the 
caps  may  be  due  to  the  condensation  of  carbonic  acid. 
We  can  only  say  of  this,  that  the  theory,  while  not  impos- 
sible, seems  to  lack  probability. 

The  reader  will  excuse  me  from  saying  anything  in 
this  chapter  about  the  possible  inhabitants  of  Mars.  He 


188    PLANETS  AND  THEIR  SATELLITES 

knows  just  as  much  of  the  subject  as  I  do,  and  that  is 
nothing  at  all. 

The  Satellites  of  Mars 

No  discovery  more  surprised  the  whole  world  than  that 
of  two  satellites  of  Mars  by  Professor  Asaph  Hall,  at 
the  Naval  Observatory,  in  1877.  They  had  failed  of 
previous  detection  owing  to  their  extreme  minuteness.  It 
was  not  considered  likely  that  a  satellite  could  be  so  small 
as  these  were  found  to  be,  and  so  no  one  had  taken  the 
trouble  to  make  a  careful  search  with  any  great  telescope. 
But,  when  once  discovered,  they  were  found  to  be  by  no 
means  difficult  objects.  Of  course  the  ease  with  which 
they  can  be  seen  depends  on  the  position  of  Mars  both  in 
its  orbit  and  with  respect  to  the  earth.  They  are  never 
visible  except  when  the  planet  is  near  its  opposition.  At 
each  opposition  they  may  be  observed  for  a  period  of 
three,  four,  or  even  six  months,  according  to  circum- 
stances. At  an  opposition  near  perihelion  they  may  be 
seen  with  a  telescope  of  less  than  twelve  inches  diameter ; 
how  small  a  one  will  show  them  depends  on  the  skill  of  the 
observer,  and  the  pains  he  takes  to  cut  off  the  light  of 
the  planet  from  his  eye.  Generally  a  telescope  ranging 
from  twelve  to  eighteen  inches  in  diameter  is  necessary. 
The  difficulty  in  seeing  them  arises  entirely  from  the 
glare  of  the  planet.  Could  this  be  eliminated  they  could 
doubtless  be  seen  with  much  smaller  instruments.  Owing 
to  the  glare,  the  outer  one  is  much  easier  to  see  than  the 
inner  one,  although  the  inner  one  is  probably  the  brighter 
of  the  twOo 


THE    SATELLITES    OF    MARS          189 

Professor  Hall  assigned  the  name  Delmos  to  the  outer 
and  Phobos  to  the  inner,  these  being  the  attendants  of 
Mars  in  ancient  mythology.  Phobos  has  the  remark- 
able peculiarity  that  it  revolves  around  the  planet  in 
less  than  nine  hours,  making  its  period  the  shortest  of 
any  yet  known  in  the  solar  system.  This  is  little  more 
than  one  third  the  time  of  the  planet's  rotation  on  its 
axis.  The  consequence  of  this  is  that,  to  the  inhabitants 
of  the  planet,  its  nearest  moon  rises  in  the  west  and 
sets  in  the  east. 

Deimos  performs  its  revolution  in  30  hours  18  minutes. 
The  result  of  this  rapid  motion  is  that  some  two  days 
must  elapse  between  its  rising  and  setting. 

Phobos  is  only  8,700  miles  from  the  surface  of  the 
planet.  It  must  therefore  be  an  interesting  object  to  the 
inhabitants  of  Mars,  if  they  have  telescopes. 

In  size  these  bodies  are  the  smallest  visible  to  us  in  the 
solar  system,  with  the  possible  exception  of  Eros  and 
possibly  some  others  of  the  fainter  asteroids.  From  Pro- 
fessor Pickering's  photometric  estimates  their  diameter 
was  estimated  to  be  not  very  different  from  seven  miles. 
Their  apparent  size  as  we  view  them  is  therefore  not  very 
different  from  that  of  a  small  apple  hanging  over  the 
city  of  Boston,  and  seen  with  a  telescope  from  the  city 
of  New  York.  In  this  respect  they  form  a  singular  con- 
trast to  nearly  or  quite  all  of  the  other  satellites,  which 
are  generally  a  thousand  miles  or  more  in  diameter.  The 
one  exception  to  this  is  the  fifth  satellite  of  Jupiter,  to  be 
described  in  the  chapter  on  Jupiter  and  its  satellites. 
Although  this  is  much  less  than  a  thousand  miles  in  diam- 


190    PLANETS  AND  THEIR  SATELLITES 

eter,  it  must  considerably  exceed  the  satellites  of  Mars 
in  size. 

The  satellites  have  been  most  useful  to  the  astronomer 
in  enabling  him  to  learn  the  exact  mass  of  Mars.  How 
this  is  done  will  be  explained  in  a  subsequent  chapter, 
where  the  methods  of  weighing  the  planets  are  set  forth. 

The  satellites  also  offer  many  curious  and  difficult 
problems  in  gravitation.  Their  orbits  seem  to  have  a 
slight  eccentricity,  and  the  position  of  the  planes  in  which 
they  revolve  changes  in  consequence  of  the  bulging  of 
the  planet  at  its  equator,  produced  by  its  rotation.  The 
calculation  of  these  changes  and  their  comparison  with 
observations  have  opened  up  a  field  of  research  in  which 
Professor  Hermann  Struve,  now  of  the  University  of 
Koenigsberg,  Germany,  has  taken  a  leading  part. 


V 

THE  GROUP  OF  MINOR  PLANETS 

THE  seeming  gap  in  the  solar  system  between  the 
orbits  of  Mars  and  Jupiter  naturally  attracted  the  at- 
tention of  astronomers  as  soon  as  the  distances  of  the 
planets  had  been  accurately  laid  down.  It  became  very 
striking  when  Bode  announced  his  law.  There  was  a 
row  of  eight  numbers  in  regular  progression,  and  every 
number  but  one  represented  the  distance  of  a  planet. 
That  one  place  was  vacant.  Was  the  vacancy  real,  or 
was  it  only  because  the  planet  which  filled  it  was  so  small 
that  it  had  escaped  notice? 

This  question  was  settled  by  Piazzi,  an  Italian  as- 
tronomer who  had  a  little  observatory  in  Palermo  in 
Sicily.  He  was  an  ardent  observer  of  the  heavens,  and 
was  engaged  in  making  a  catalogue  of  all  the  stars 
whose  positions  he  could  lay  down  with  his  instrument. 
On  January  1,  1801,  he  inaugurated  the  new  century 
by  finding  a  star  where  none  had  existed  before;  and 
this  star  soon  proved  to  be  the  long-looked-for  planet.  It 
received  the  name  of  Ceres,  the  goddess  of  the  wheat 
field. 

It  was  a  matter  of  surprise  that  the  planet  should  be 
so  small;  and  when  its  orbit  became  known  it  proved  to 
be  very  eccentric.  But  new  revelations  were  soon  to 
come.  Before  the  new  planet  had  completed  a  revolu- 


192    PLANETS  AND  THEIR  SATELLITES 

tion  after  its  discovery,  Dr.  Olbers,  a  physician  of  Bre- 
men, who  employed  his  leisure  in  astronomical  observa- 
tions and  researches,  found  another  planet  revolving  in 
the  same  region.  Instead  of  one  large  planet  there  were 
two  small  ones.  He  suggested  that  these  might  be 
fragments  of  a  shattered  planet,  and  that,  if  so,  more 
would  probably  be  found.  The  latter  part  of  the  con- 
jecture proved  true.  Within  the  next  three  years  two 
more  of  these  little  bodies  were  discovered,  making  four 
in  all. 

Thus  the  matter  remained  for  some  forty  years. 
Then,  in  1845,  Hencke,  a  German  observer,  found  a 
fifth  planet.  The  year  following  a  sixth  was  added, 
and  then  commenced  the  curious  series  of  discoveries 
which,  proceeding  year  by  year,  are  now  carrying  the 
number  known  rapidly  past  five  hundred. 

Hunting  Asteroids 

Up  to  1890  these  bodies  had  been  found  by  a  few 
observers  who  devoted  especial  attention  to  the  search, 
and  caught  the  tiny  stars  as  the  hunter  does  game.  They 
would  lay  traps,  so  to  speak,  by  mapping  the  many  small 
stars  in  some  small  region  of  the  sky  near  the  ecliptic, 
familiarise  themselves  with  their  arrangement,  and  then 
watch  for  an  intruder.  Whenever  one  appeared,  it  was 
found  to  be  one  of  the  group  of  minor  planets,  and  the 
hunter  put  it  into  his  bag. 

Quite  a  succession  of  planet-hunters  appeared,  some 
of  them  little  known  for  any  other  astronomical  work. 
The  most  successful  of  these  in  the  fifties  was  Gold- 


HUNTING    ASTEROIDS  193 

schmidt,  of  Paris,  a  jeweller  if  I  mistake  not.  Three 
were  discovered  by  Professor  James  Ferguson  at  the 
Washington  Observatory.  Palisa,  of  Vienna,  made  a 
record  for  himself  in  this  work.  In  this  country  Pro- 
fessors C.  H.  F.  Peters,  of  Clinton,  and  James  C.  Wat- 
son, of  Ann  Arbor,  were  very  successful.  The  last  three 
observers  carried  the  number  above  the  two  hundred 
mark. 

About  1890  the  photographic  art  was  found  to  offer 
a  much  easier  and  more  effective  means  of  finding  these 
objects.  The  astronomer  would  point  his  telescope  at 
the  sky  and  photograph  the  stars  with  a  pretty  long 
exposure,  perhaps  half  an  hour,  more  or  less.  The  stars 
proper  would  be  taken  on  the  negative  as  small  round 
dots.  But  if  a  planet  happened  to  be  among  them  it 
would  be  in  motion,  and  thus  its  picture  would  be  taken 
as  a  short  line,  and  not  as  a  dot.  Instead  of  scanning 
the  heavens  the  observer  had  only  to  scan  his  photo- 
graphic plate,  a  much  easier  task,  because  the  planet 
could  be  recognised  at  once  by  its  trail. 

Recently  a  dozen  or  more  of  these  bodies  have  been 
found  nearly  every  year.  Of  course  the  unknown  ones 
are  smaller  and  more  difficult  to  find  as  the  years  elapse. 
But  there  is  as  yet  no  sign  of  a  limit  to  the  number. 
Most  of  those  newly  discovered  are  very  minute,  yet  the 
number  seems  to  increase  with  their  smallness.  Even 
the  larger  of  these  bodies  are  so  small  that  they  appear 
only  as  star-like  points  in  ordinary  telescopes,  and 
their  disks  are  hard  to  make  out  even  with  the  most 
powerful  instruments.  So  far  as  can  be  determined, 


194    PLANETS  AND  THEIR  SATELLITES 

the  diameters  of  the  largest  ones,  naturally  the  earliest 
discovered,  are  only  three  or  four  hundred  miles.  The 
size  of  the  smallest  can  be  inferred  only  in  a  rough 
way  from  their  brightness.  They  may  be  twenty  or 
thirty  miles  in  diameter. 

Orbits  of  the  Asteroids 

The  orbits  of  these  bodies  are  for  the  most  part  very 
eccentric.  In  the  case  of  Polyhymnia,  the  eccentricity 
is  about  0.33,  which  means  that  at  perihelion  it  is  one 
third  nearer  the  sun  than  its  mean  distance,  and  at  aphe- 
lion one  thirpl  more.  It  happens  that  its  mean  distance 
is  just  about  three  astronomical  units;  its  least  distance 
from  the  sun  is  therefore  two,  its  greatest  four,  or  twice 
as  great  as  the  least. 

The  large  inclination  of  most  of  the  orbits  is  also 
noteworthy.  In  several  cases  it  exceeds  twenty  degrees, 
in  that  of  Pallas  it  is  twenty-eight  degrees. 

Gibers'  idea  that  these  bodies  might  be  fragments  of 
a  planet  which  had  been  shattered  by  some  explosion  is 
now  abandoned.  The  orbits  range  through  too  wide  a 
space  ever  to  have  joined,  as  they  would  have  done  if 
the  asteroids  had  once  formed  a  single  body.  In  the 
philosophy  of  our  time  these  bodies  have  been  as  we  see 
them  since  the  beginning.  On  the  theory  of  the  nebular 
hypothesis  the  matter  of  all  the  planets  once  formed 
rings  of  nebulous  substance  moving  round  the  sun.  In 
the  case  of  all  the  other  planets  the  material  of  these 
rings  gradually  gathered  around  the  densest  point  of 
the  ring,  thus  agglomerating  into  a  single  body.  But 


GROUPING    OF    THE    ASTEROIDS       195 

it  is  supposed  that  the  ring  forming  the  minor  plan- 
ets did  not  collect  in  this  way,  but  separated  into  in- 
numerable fragments. 

Groupings  of  the  Orbits 

There  is  a  curious  feature  of  the  orbits  of  these  bodies 
which  may  throw  some  light  on  the  question  of  their 
origin.  I  have  explained  that  the  planetary  orbits  are 
nearly  exact  circles,  but  that  these  circles  are  not  cen- 
tred on  the  sun.  Now  imagine  ourselves  to  look  down 
upon  the  solar  system  from  an  immense  height,  and  sup- 
pose that  the  orbits  of  the  minor  planets  were  visible 
as  finely  drawn  circles.  These  circles  would  appear  to 
interlace  and  cross 
each  other  like  an 
intricate  network, 
filling  a  broad  ring 
of  which  the  outer 
diameter  would  be 
nearly  or  quite 
double  the  inner 
qne. 

But    suppose    we 
could  pick  all  these      ^  saf_Sfmlia^f  &  Minor  Planet, 
circles  up,  as  if  they  into  Groups. 

were  made  of  wire, 

and  centre  them  all  on  the  sun,  without  changing  their 
size.  The  diameters  of  the  larger  ones  would  be  double 
those  of  the  smaller,  so  that  the  circles  would  fill  a  broad 
space,  as  shown  in  the  figure.  Now,  the  curious  fact  is 


196    PLANETS  AND  THEIR  SATELLITES 


^JUPITER 


that  they  would  not  fill  the  whole  space  uniformly,  but 
would  be  collected  into  distinct  groups.  These  groups 
are  shown  on  the  figures  of  their  orbits,  given  above,  and, 
on  a  different  plan,  and  more  com- 
pletely, in  the  second  figure,  which  is 
arranged  on  a  plan  explained  as  fol- 
lows: Every  planet  performs  its  revo- 
lution in  a  certain  number  of  days, 
which  is  greater  the  farther  the  planet 
from  the  sun.  Since  the  complete  cir- 
cumference of  the  orbit  measures 
1,296,000",  it  follows  that  if  we 
divide  this  number  by  the  time  of  revo- 
lution, the  quotient  will  show  through 
what  angle,  on  the  average,  the  planet 
moves  along  its  orbit  in  one  day.  This 
angle  is  called  the  mean  motion  of  the 
planet.  In  the  case  of  the  minor 
planets  it  ranges  from  400"  to  more 
than  1,000",  being  greater  the  shorter 
the  time  of  revolution  and  the  nearer 
the  planet  is  to  the  sun. 

Now  we  draw   a   vertical  line   and 
mark  off  on  it  values  of  the  mean  mo- 
tion, from  four  hundred  to  one  thou- 
sand   seconds,    differing    by    ten    sec- 
onds.     Between    each   pair   of   marks 
we  make  as  many  points  as  there  are  planets  having 
mean   motions  between   the   limits.      For   example,  be- 
tween 550"  and  560"  there  are  three  dots.     This  means 


750 


JJUPITER 


FIG.  36. — Distribti- 
tion  of  the  Or- 
bits of  the  Minor 
Planets. 


GROUPING    OF    THE    ASTEROIDS      197 

that  there  are  three  planets  having  mean  motions  between 
550"  and  560".  There  are  also  four  planets  between 
560"  and  570",  and  one  between  570"  and  580".  Then 
there  are  no  more  till  we  pass  610",  when  we  find  six 
planets  between  610"  and  620",  followed  by  a  multitude 
of  others. 

Examining  the  diagram  we  are  able  to  distinguish 
five  or  six  groups.  The  outermost  one  is  between  400" 
and  460",  and  is  nearest  to  Jupiter.  The  times  of  revo- 
lution are  not  far  from  eight  years.  Then  there  is  a  wide 
gap  extending  to  560",  when  we  have  a  group  of  ten 
planets  between  540"  and  580".  From  this  point  down- 
wards the  planets  are  more  numerous,  but  we  find  very 
sparse  or  empty  points  at  700",  750",  and  900".  Now 
the  most  singular  feature  of  the  case  is  that  these  empty 
spaces  are  those  in  which  the  motion  of  a  planet  would 
have  a  simple  relation  to  that  of  Jupiter.  A  planet  with 
a  mean  motion  of  900"  would  make  its  circuit  round  the 
sun  in  one  third  the  time  that  Jupiter  does ;  one  of  600 
in  half  the  time;  one  of  750"  in  two  fifths  of  the  time. 
It  is  a  law  of  celestial  mechanics  that  the  orbits  of  planets 
having  these  simple  relations  to  another  undergo  great 
changes  in  the  course  of  time  from  their  action  on  each 
other.  It  was  therefore  supposed  by  Kirk  wood,  who  first 
pointed  out  these  gaps  in  the  series,  that  they  arose  be- 
cause a  planet  within  them  could  not  keep  its  orbit  per- 
manently. But  it  is  curious  that  there  is  no  gap,  but  on 
the  contrary,  a  group  of  planets  whose  mean  motion  is 
nearly  two  thirds  of  that  of  Jupiter.  Hence  the  view  is 
doubtful. 


198    PLANETS  AND  THEIR  SATELLITES 

The  Most  Curious  of  the  Asteroids 

One  of  these  bodies  is  so  exceptional  as  to  attract  our 
special  attention.  All  the  hundreds  of  minor  planets 
known  up  to  1898  moved  between  the  orbits  of  Mars 
and  Jupiter.  But  in  the  summer  of  that  year  Witt,  of 
Berlin,  found  a  planet  which,  at  perihelion,  came  far 
within  the  orbit  of  Mars — in  fact  within  fourteen  million 
miles  of  the  orbit  of  the  earth.  He  named  it  Eros. 
The  eccentricity  of  its  orbit  is  so  great  that  at  aphelion 
the  planet  is  Considerably  outside  the  orbit  of  Mars. 
Moreover  the  two  orbits,  that  of  the  planet  and  of  Mars, 
pass  through  each  other  like  two  links  of  a  chain,  so 
that  if  the  orbits  were  represented  of  wire  they  would 
hang  together. 

Owing  to  the  inclination  of  its  orbit,  this  planet 
seems  to  wander  far  outside  the  limits  of  the  zodiac. 
When  nearest  the  earth,  as  it  was  in  1900,  it  was  for  a 
time  so  far  north  that  it  never  set  in  our  middle  lati- 
tudes, and  passed  the  meridian  north  of  the  zenith.  This 
peculiarity  of  its  motion  was  doubtless  one  reason  why 
it  was  not  found  sooner.  During  its  near  approach  in 
the  winter  of  1900-'01  it  was  closely  scrutinised  and 
found  to  vary  in  brightness  from  hour  to  hour.  Care- 
ful observation  showed  that  these  changes  went  through 
a  regular  period  of  about  two  and  a  half  hours.  At 
this  interval  it  would  fade  away  a  little  with  great  uni- 
formity. Some  observers  maintained  that  it  was  fainter 
at  every  alternate  minimum  of  light,  so  that  the  real 
period  was  five  hours.  It  was  supposed  that  this  indi- 


MOST  CURIOUS  OF  THE  ASTEROIDS  199 

cated  that  the  object  was  really  made  up  of  two  bodies 
revolving  round  each  other — perhaps  actually  joined 
into  one.  But  it  seems  more  likely  that  the  variations 
of  light  were  due  to  there  being  light  and  dark  regions 
on  the  surface  of  the  little  planet,  which  therefore 
changed  in  brightness  according  as  bright  or  dark 
"regions  predominated  on  the  surface  of  the  hemisphere 
turned  toward  us.  The  case  was  made  perplexing  by 
the  gradual  disappearance  of  the  variations  after  they 
had  been  well  established  by  months  of  observation. 
There  seems  to  be  some  mystery  in  the  constitution  of 
this  body. 

From  a  scientific  point  of  view  Eros  is  most  interesting 
because,  coming  so  near  the  earth  from  time  to  time, 
its  distance  may  be  measured  with  great  precision,  and 
the  distance  of  the  sun  as  well  as  the  dimensions  of  the 
whole  solar  system  thus  fixed  with  greater  exactness 
than  by  any  other  method.  Unfortunately,  the  nearest 
approaches  occur  only  at  very  long  intervals.  What 
is  most  tantalising  is  that  there  was  such  an  approach  in 
1892  before  the  object  was  recognised.  At  that  time  it 
was  photographed  a  number  of  times  at  the  Harvard 
Observatory,  but  was  lost  in  the  mass  of  stars  by  which 
it  was  surrounded.  Its  distance  was,  astronomically,  only 
sixteen  hundredths,  or  some  fifteen  millions  of  miles, 
while  the  nearest  approaches  of  Mars  are  nearly  forty 
millions.  There  will  not  be  another  approach  so  near  for 
more  than  sixty,  perhaps  not  for  more  than  a  hundred 
years. 

In  1900  it  approached  the  earth  within  about  thirty 


200    PLANETS  AND  THEIR  SATELLITES 

millions  of  miles,  and  a  combined  effort  was  made  at 
various  observatories  to  lay  down  its  exact  position  from 
night  to  night  among  the  stars  by  photography,  with  a 
view  to  determining  its  parallax.  But  the  planet  was 
faint,  the  observations  were  difficult,  and  it  is  not  yet 
known  what  measure  of  success  was  reached. 

Variations  of  light  which  might  be  due  to  a  rotation 
on  their  axes  have  been  suspected  in  the  case  of  other 
asteroids  besides  Eros,  but  nothing  has  yet  been  settled. 


VI 

JUPITER  AND  ITS   SATELLITES 

JUPITER,  the  "giant  planet,"  is,  next  the  sun,  the 
largest  body  of  the  solar  system.  It  is,  in  fact,  more  than 
three  times  as  large,  and  about  three  times  as  massive  as 
all  the  other  planets  put  together.  Yet,  such  is  the  pre- 
ponderating mass  of  our  central  luminary  that  the  mass 
of  Jupiter  is  less  than  one  thousandth  part  that  of  the 
sun. 

This  planet  is  in  opposition  in  September,  1903,  Octo- 
ber, 1904,  November,  1905,  and  so  on  for  several  years 
afterward,  about  a  month  later  every  year.  Near  the 
time  of  opposition  it  may  easily  be  recognised  in  the  even- 
ing sky,  both  by  its  brightness  and  its  colour.  It  is  then, 
next  to  Venus,  the  brightest  star-like  object  in  the 
heavens.  It  can  easily  be  distinguished  from  Mars  by  its 
whiter  colour.  If  we  look  at  it  with  a  telescope  of  the 
.smallest  size,  even  with  a  good  ordinary  spy-glass,  we 
shall  readily  see  that  instead  of  being  a  bright  point,  like 
a  star,  it  is  a  globe  of  very  appreciable  dimensions.  We 
shall  also  see  what  look  like  two  shadowy  belts  crossing 
the  disk.  These  were  noticed  and  pictured  two  hundred 
years  ago  by  Huygens.  As  greater  telescopic  power  was 
used  it  was  found  that  these  seeming  belts  resolved  them- 
selves into  very  variegated  cloud-like  forms,  and  that 
they  vary,  not  only  from  month  to  month,  but  even  from 


202    PLANETS  AND  THEIR  SATELLITES 

night  to  night.  By  careful  observation  on  the  aspect 
which  they  present  from  hour  to  hour,  and  from  night  to 
night,  it  was  found  that  the  planet  rotates  on  its  axis 
in  about  9  hours  55  minutes.  The  astronomer  may  there- 
fore in  the  course  of  a  single  night  see  every  part  of  the 
surface  of  the  planet  presented  to  his  view  in  succession. 

Two  features  presented  by  the  planet  will  at  once 
strike  the  careful  observer  with  the  telescope.  One  of 
these  is  that  the  disk  does  not  seem  uniformly  bright,  but 
gradually  shades  off  near  the  limb.  The  latter,  instead 
of  being  bright  and  hard  is  somewhat  soft  and  diffuse. 
In  this  respect  the  appearance  forms  quite  a  contrast  to 
that  presented  by  the  moon  or  Mars.  The  shading  off 
toward  the  edge  is  sometimes  attributed  to  a  dense  atmos- 
phere surrounding  the  planet.  While  this  is  possible, 
it  is  by  no  means  certain. 

The  other  feature  to  which  we  allude  is  an  ellipticity  of 
the  disk.  Instead  of  being  perfectly  round,  the  planet 
is  flattened  at  the  poles,  like  our  earth,  but  in  a  much 
greater  degree.  The  most  careful  observer,  viewing  the 
earth  from  another  planet,  would  see  no  deviation  from 
the  spherical  form,  but,  viewing  Jupiter,  the  deviation  is 
very  perceptible.  This  is  owing  to  its  rapid  rotation  on 
its  axis,  which  causes  its  equatorial  regions  to  bulge  out, 
as,  to  a  smaller  degree,  in  the  case  of  the  earth. 

Surface  of  Jupiter 

The  features  of  Jupiter,  as  we  see  them  with  a  tele- 
scope, are  almost  as  varied  as  those  of  the  clouds  which 
we  see  in  our  atmosphere.  There  are  commonly  elon- 


SURFACE    OF    JUPITER  203 

gated  strata  of  clouds,  apparently  due  to  the  same  cause 
that  produces  stratified  clouds  on  the  earth,  namely,  cur- 
rents of  air.  Among  these  clouds  round  white  spots  are 
frequently  seen.  The  clouds  are  sometimes  of  a  rosy 
tinge,  especially  those  near  the  equator.  They  are 
darkest  and  most  strongly  marked  in  middle  latitudes, 
both  north  and  south  of  the  equatorial  regions.  It  is 
this  that  produces  the  appearance  of  dark  belts  in  a  small 
telescope. 

The  appearance  of  Jupiter  is,  in  almost  every  point, 
very  different  from  that  of  Mars  or  Venus.  Comparing 
it  with  Mars,  the  most  strongly  marked  difference  con- 
sists in  the  entire  absence  of  permanent  features.  Maps 
of  Mars  may  be  constructed  and  their  correctness  tested 
by  observations  generation  after  generation,  but  owing 
to  the  absence  of  permanence,  no  such  thing  as  a  map  of 
Jupiter  is  possible. 

Notwithstanding  this  lack  of  permanence,  features 
have  been  known  to  endure  through  a  number  of  years, 
and  some  of  them  may  be  permanent.  The  most  remark- 
able of  these  was  the  great  red  spot,  which  appeared  in 
middle  latitudes,  on  the  southern  hemisphere  of  the  planet, 
about  the  year  1878.  For  several  years  it  was  a  very 
distinct  object,  readily  distinguished  by  its  colour.  After 
ten  years  it  began  to  fade  away,  but  not  at  a  uniform 
rate.  Sometimes  it  would  seem  to  disappear  entirely, 
then  would  brighten  up  once  more.  These  changes  con- 
tinued but,  since  1892,  faintness  or  invisibility  has  been 
the  rule.  If  the  spot  finally  disappeared,  it  was  in  so  un- 
certain a  way  that  no  exact  date  for  the  last  observation 


CONSTITUTION    OF    JUPITER          205 

of  it  can  be  given.     Some  observers  with  good  eyes  still 
report  it  to  be  visible  from  time  to  time. 

Constitution  of  Jupiter. 

The  question  of  the  constitution  of  this  curious  planet 
is  still  an  unsettled  one.  There  is  no  one  hypothesis  that 
readily  explains  all  the  facts,  which  suggest  many  points, 
but  prove  few,  unless  negatively. 

Perhaps  the  most  remarkable  feature  of  the  planet  is 
its  small  density.  Its  diameter  is  about  eleven  times  that 
of  the  earth.  It  follows  that,  in  volume,  it  must  exceed 
the  earth  more  than  thirteen  hundred  times.  But  its 
mass  is  only  a  little  more  than  three  hundred  times  that 
of  the  earth.  It  follows  from  this  that  its  density  is  much 
less  than  that  of  the  earth ;  as  a  matter  of  fact,  it  is  only 
about  one  third  greater  than  the  density  of  water.  A 
simple  computation  shows  that  the  force  of  gravity  at  its 
surface  is  between  two  and  three  times  that  at  the  surface 
of  the  earth.  Under  this  enormous  gravitation  we  might 
suppose  its  interior  to  be  enormously  compressed,  and  its 
density  to  be  great  in  comparison.  Such  would  certainly 
be  the  case  were  it  made  up  of  solid  or  fluid  matter  of  the 
same  kind  that  composes  the  surface  of  the  earth.  From 
this  fact  alone  the  conclusion  would  be  that  its  outer  por- 
tions at  least  were  composed  of  aeriform  matter.  But 
how  reconcile  this  form  with  the  endurance  of  the  red 
spot  through  twenty-five  years?  This  is  the  real  diffi- 
culty of  the  case. 

Nevertheless,  the  hypothesis  is  one  which  we  are  forced 
to  accept  without  great  modification.  Besides  the  evi- 


206    PLANETS  AND  THEIR  SATELLITES 

dence  of  vapour  as  shown  by  the  constantly  changing 
aspect  of  the  planet,  we  have  another  almost  conclusive 
piece  of  evidence  in  the  law  of  rotation.  It  is  found  that 
Jupiter  resembles  the  sun  in  that  its  equatorial  region 
rotates  in  less  time  than  the  regions  north  of  middle  lati- 
tude, although  the  circuit  they  have  to  make  is  longer. 
This  is  probably  a  law  of  rotation  of  gaseous  bodies  in 
general.  It  seems,  therefore,  that  Jupiter  has  a  greater 
or  less  resemblance  to  the  sun  in  its  physical  constitution, 
a  view  which  quite  corresponds  with  its  aspect  in  the  tele- 
scope. The  difference  in  the  time  of  rotation  at  the  equa- 
tor and  in  middle  latitudes  is,  so  far  as  we  yet  know,  about 
five  minutes.  That  is  to  say,  the  equatorial  region  rotates 
in  nine  hours  fifty  minutes  and  those  in  middle  latitudes  in 
nine  hours  fifty-five  minutes.  This  corresponds  to  a  dif- 
ference of  velocity  of  the  motion  between  the  two  amount- 
ing to  about  two  hundred  miles  an  hour;  a  seemingly 
impossible  difference  were  the  surface  liquid. 

It  is  a  singular  fact  that  no  well-defined  law  of  rotation 
in  different  latitudes  has  yet  been  made  out,  as  has  been 
done  in  the  case  of  the  sun.  Were  we  to  accept  the  re- 
cults  of  the  meagre  observations  at  our  disposal  we  might 
be  led  to  the  conclusion  that  the  difference  of  time  is 
not  a  gradually  varying  quantity,  as  we  go  from  the 
equator  toward  the  poles,  but  that  the  change  of  five 
minutes  occurs  very  near  a  certain  latitude  and  almost 
suddenly.  But  we  cannot  assume  this  to  be  the  case 
without  more  observations  than  are  yet  on  record.  The 
subject  is  one  of  which  an  accurate  investigation  is 
greatly  to  be  desired. 


CONSTITUTION    OF    JUPITER          207 

Yet  another  resemblance  between  Jupiter  and  the  sun 
is  that  they  are  both  brighter  in  the  centre  of  their  disk 
than  toward  the  circumference.  In  the  case  of  Jupiter, 
the  shading  off  is  very  well  marked.  The  extreme  cir- 
cumference especially  is  more  softened  than  that  of  any 
of  the  other  planets. 

The  apparent  resemblance  between  the  surfaces  of 
these  bodies,  taken  in  connection  with  the  brightness  of 
the  planet,  has  led  to  the  question  whether  Jupiter  may 
not  be,  in  whole  or  in  part,  self-luminous.  This  again  is 
a  question  which  needs  investigation.  The  idea  that  the 
planet  can  emit  much  light  of  its  own  seems  to  be  nega- 
tived by  the  fact  that  the  satellites  completely  disappear 
when  they  pass  into  its  shadow.  We  may  therefore  say 
with  entire  certainty  that  Jupiter  does  not  give  enough 
light  to  enable  us  to  see  a  satellite  by  that  light  alone. 
We  can  hardly  suppose  that  this  would  be  the  case  if  the 
satellite  received  one  per  cent  as  much  light  from  the 
planet  as  it  does  from  the  sun.  It  is  also  found  that  the 
light  which  Jupiter  sends  out  is  somewhat  less  than 
that  which  it  receives  from  the  sun.  That  is  to  say,  all 
the  light  which  it  gives  out,  when  estimated  in  quantity, 
may  be  reflected  light,  without  supposing  the  planet 
brighter  than  white  bodies  on  the  surface  of  the  earth. 
But  this  still  leaves  open  the  question  whether  the  white 
spots,  sometimes  so  much  brighter  than  the  rest  of  the 
planet,  may  not  give  us  more  light  than  can  fall  upon 
them.  This  also  is  a  question  not  yet  investigated. 

The  hypothesis  which  best  lends  itself  to  all  the  facts 
seems  to  be  that  the  planet  has  a  solid  nucleus,  of  which 


208    PLANETS  AND  THEIR  SATELLITES 

the  density  may  be  as  great  as  that  of  the  earth  or  any 
other  solid  planet,  and  that  the  small  average  density 
of  the  entire  mass  is  due  to  the  vapourous  character  of  the 
matter  which  surrounds  this  nucleus.  In  all  probability 
the  nucleus  is  at  a  very  high  temperature,  even  ap- 
proximating that  at  the  surface  of  the  sun,  but  this 
temperature  gradually  diminishes  as  we  ascend  through 
the  gaseous  atmosphere,  as  we  suppose  to  be  the  case 
with  the  sun ;  hence  it  may  happen  that,  at  the  surface, 
none  of  the  material  that  we  see  is  at  a  high  enough 
temperature  to  radiate  a  sensible  amount  of  heat. 

On  the  whole  we  may  describe  Jupiter  as  a  small  sun 
of  which  the  surface  has  cooled  off  till  it  no  longer  emits 
light. 

The  Satellites  of  Jupiter 

When  Galileo  first  turned  his  little  telescope  on  the 
planet  Jupiter  he  was  delighted  and  surprised  to  find  it 
accompanied  by  four  minute  companions.  Watching 
them  from  night  to  night,  he  found  them  to  be  in  rev- 
olution around  their  central  body  as,  upon  the  theory 
not  fully  accepted  in  his  time,  the  planets  revolve 
around  the  sun.  This  remarkable  resemblance  to  the 
solar  system  was  a  strong  point  in  favor  of  the  Coper- 
nican  Theory. 

These  bodies  can  be  seen  with  a  common  spy-glass,  or 
even  a  good  opera  glass.  It  has  even  been  supposed  that 
good  eyes  sometimes  see  them  without  optical  assistance. 
They  are  certainly  as  bright  as  the  smallest  stars  visible 
to  the  naked  eye,  yet  the  glare  of  the  planet  would  seem 
to  be  an  insuperable  obstacle  to  their  visibility,  even  to 


THE    SATELLITES    OF    JUPITER       209 

the  keenest  vision.  A  story  has  been  told,  by  Arago,  I 
think,  of  a  woman  who  professed  to  be  able  to  see  them  at 
any  time  and  even  pointed  out  their  positions.  It  was 
found,  however,  that  she  described  them  as  on  the  op- 
posite side  of  the  planet  to  that  on  which  they  were  really 
situated.  It  was  then  found,  or  inferred,  that  she  took 
the  positions  from  an  astronomical  ephemeris,  in  which 
diagrams  of  them  were  given,  but  in  which  the  pictures 
were  made  upside  down  in  order  that  the  satellites  might 
be  seen  as  in  an  ordinary  inverting  telescope.  But  it 
seems  quite  likely  that,  when  the  two  outer  satellites 
chance  to  be  nearly  in  the  same  straight  line,  they  may 
be  visible  by  their  combined  light. 

From  the  measures  of  Barnard  it  may  be  inferred  that 
these  bodies  range  somewhere  between  two  and  three  thou- 
sand miles  in  diameter.  Hence,  they  do  not  differ  greatly 
from  our  moon  in  size. 

Only  four  satellites  were  known  until  1892 ;  then  Bar- 
nard, with  the  great  Lick  telescope,  discovered  a  fifth, 
much  nearer  the  planet  than  the  four  others.  It  makes 
a  revolution  in  a  little  less  than  twelve  hours,  the  short- 
jest  periodic  time  known  except  that  of  the  inner  satellite 
of  Mars.  Still,  however,  it  is  a  little  longer  than  the 
rotation  time  of  the  planet.  The  next  outer  one,  or  the 
innermost  of  the  four  previously  known,  still  called  the 
first  satellite,  revolves  in  about  one  day  eighteen  and  a 
half  hours,  while  the  outer  one  requires  nearly  seventy 
days  to  perform  its  circuit. 

In  its  visibility  the  fifth  satellite  is  the  most  difficult 
known  object  in  the  solar  system.  Through  only  a  few 


210    PLANETS  AND  THEIR  SATELLITES 

of  the  most  powerful  telescopes  of  the  world  has  it  ever 
certainly  been  seen  by  the  human  eye.  Its  orbit  is  de- 
cidedly eccentric.  Owing  to  the  ellipticity  of  the  planet, 
it  possesses  the  remarkable  peculiarity  that  its  major  axis, 
and,  therefore,  the  perihelion  point  of  its  orbit,  performs 
a  complete  revolution  in  about  a  year. 

It  has  sometimes  been  questioned  whether  these  satel- 
lites are  round  bodies,  like  the  planets  and  most  other 
satellites.  Some  observers,  especially  Barnard  and  W.  H. 
Pickering,  noticed  curious  changes  in  the  form  of  the 
first  satellite  as  it  was  crossing  the  surface  of  the  planet. 
At  one  time  it  looked  like  a  double  body.  But  Barnard, 
by  careful  and  repeated  study,  showed  that  this  appear- 
ance was  partly  due  to  the  varying  shade  of  the  back- 
ground on  which  the  satellite  was  seen  projected  upon 
the  planet,  and  partly  to  the  differences  in  the  shade  of 
various  parts  of  the  satellite  itself. 

During  their  course  around  the  planet  these  bodies 
present  many  interesting  phenomena,  which  can  be  ob- 
served with  a  moderate  sized  telescope.  These  are  their 
eclipses  and  transits.  Of  course  Jupiter,  like  any  other 
opaque  body,  casts  a  shadow.  As  the  satellites  make 
their  round  they  nearly  always  pass  through  the  shadow 
during  that  part  of  their  course  which  is  beyond  the 
planet.  Exceptions  sometimes  occur  in  the  case  of  the 
fourth  and  most  distant  satellite,  which  may  pass  above 
or  below  the  shadow,  as  our  moon  passes  above  or  below 
that  of  the  earth.  When  a  satellite  enters  the  shadow,  it 
is  seen  to  fade  away  gradually,  and  finally  to  disappear 
from  sight  altogether. 


THE    SATELLITES    OF    JUPITER 

For  the  same  reason  the  satellites  generally  pass  across 
the  disk  of  the  planet  in  that  part  of  their  course  which 
lies  on  this  side  of  it.  The  general  rule  is  that,  when  a 
satellite  has  impinged  on  the  planet,  it  looks  brighter 
than  the  latter,  owing  to  the  darkness  of  the  planet's  limb. 
But,  as  it  approaches  the  central  regions,  it  may  look 
darker  than  the  background  of  the  planet.  Of  course 
this  does  not  arise  from  any  change  in  the  brightness  of 
the  satellite,  but  only  from  the  fact,  already  mentioned, 
that  the  planet  is  brighter  in  its  central  regions  than  at 
its  limb. 

Yet  more  interesting  and  beautiful  is  the  shadow  of  a 
satellite  which,  under  such  circumstances,  may  often  be 
seen  upon  the  planet,  looking  like  a  black  body  crossing 
alongside  the  satellite  itself.  Such  a  shadow  is  shown  in 
the  picture  of  Jupiter  on  page  204. 

The  phenomena  of  Jupiter's  satellites,  including  their 
transits  and  those  of  their  shadows,  are  all  predicted  in 
the  astronomical  ephemerides,  so  that  an  observer  can 
always  know  when  to  look  for  an  eclipse  or  transit. 

The  eclipses  of  the  inner  of  the  four  older  satellites 
occur  at  intervals  of  less  than  two  days.  By  noting  their 
times,  an  observer  in  unknown  regions  of  the  earth  can 
determine  his  longitude  more  easily  than  by  any  other 
method.  He  has  first  to  determine  the  error  of  his  watch 
on  local  time  by  certain  simple  astronomical  observations, 
quite  familiar  to  astronomers  and  navigators.  He  thus 
finds  the  local  time  at  which  an  eclipse  of  the  satellite 
takes  place.  He  compares  this  with  the  time  predicted 
in  the  ephemeris.  The  difference  gives  his  longitude 


PLANETS  AND  THEIR  SATELLITES 

according  to  the  system  set  forth  in  our  chapter  on  Time 
and  Longitude. 

The  principal  drawback  of  this  method  is  that  it  is 
not  very  accurate.  Observations  of  the  time  of  such  an 
eclipse  are  doubtful  to  a  large  fraction  of  a  minute.  This 
corresponds  to  15  minutes  of  longitude,  or  15  nautical 
miles  at  the  equator.  In  the  polar  regions  the  effect  of 
the  error  is  much  smaller,  owing  to  the  convergence  of 
the  meridians.  The  method  is,  therefore,  most  valuable 
to  polar  explorers. 


VII 

SATURN  AND  ITS   SYSTEM 

AMONG  the  planets,  Saturn  is  next  to  Jupiter  in  size 
and  mass.  It  performs  its  revolution  round  the  sun  in 
twenty-nine  and  a  half  years.  When  the  planet  is  visible 
the  casual  observer  will  generally  be  able  to  recognise 
it  without  difficulty  by  the  slightly  reddish  tint  of  its 
light,  and  by  its  position  in  the  heavens.  During  the 
next  few  years  it  will  be  in  opposition  first  in  summer 
and  then  in  autumn,  about  twelve  or  thirteen  days  later 
each  year.  Starting  from  August,  1903,  opposition  will 
occur  in  August  of  1904-'05,  September  of  1906-'08,  Oc- 
tober of  1909-'10,  and  so  on.  At  these  times  Saturn  will 
be  seen  each  evening  after  dark  in  the  eastern  or  south- 
eastern sky,  moving  toward  the  south  as  the  evening  ad- 
vances. It  looks  a  good  deal  like  Arcturus,  which,  for  a 
few  years  to  come,  will  be  visible  at  the  same  seasons, 
^only  high  up  in  the  south  or  southwest,  or  lower  down  in 
the  west. 

Although  Saturn  is  far  from  being  as  bright  as  Jupi- 
ter, its  rings  make  it  the  most  magnificent  object  in  the 
solar  system.  There  is  nothing  else  like  them  in  the 
heavens,  and  it  is  not  surprising  that  they  were  an 
enigma  to  the  early  observers  with  the  telescope.  To 
Galileo  they  first  appeared  as  two  handles  to  the  planet. 
After  a  year  or  two  they  disappeared  from  his  view.  We 


PLANETS  AND  THEIR  SATELLITES 

now  know  that  this  occurred  because,  owing  to  the  motion 
of  the  planet  in  its  orbit,  they  were  seen  edge-on,  and 
are  then  so  thin  as  to  be  invisible  in  a  telescope  as  imper- 
fect as  Galileo's.  But  the  disappearance  was  a  source 
of  great  embarrassment  to  the  Tuscan  philosopher,  who 
is  said  to  have  feared  that  he  had  been  the  victim  of  some 
illusion  on  the  subject,  and  ceased  to  observe  Saturn. 
He  was  then  growing  old,  and  left  to  others  the  task  of 
continuing  his  observations.  Of  course  the  handles  soon 
reappeared,  but  there  was  no  way  of  learning  what  they 
were.  After  more  than  forty  years  the  riddle  was  solved 
by  Huyghens,  the  great  Dutch  astronomer  and  physicist, 
who  announced  that  the  planet  was  surrounded  by  a  thin 
plane  ring,  nowhere  touching  it,  and  inclined  to  the 
ecliptic. 

Satellites  of  Saturn 

Besides  his  rings,  Saturn  is  surrounded  by  a  retinue  of 
eight  satellites — a  greater  number  than  any  other  planet. 
The  existence  of  a  ninth  has  been  suspected,  but  awaits 
confirmation.  They  are  very  unequal  in  size  and  dis- 
tance from  the  planet.  One,  Titan,  may  be  seen  with  a 
small  telescope ;  the  faintest,  only  in  very  powerful  ones. 

Titan  was  discovered  by  Huyghens  just  as  he  had 
made  out  the  true  nature  of  the  rings.  And  hereby 
hangs  a  little  tale  wrhich  has  only  recently  come  out 
through  the  publication  of  Huyghens's  correspondence. 
Following  a  practice  of  the  time,  the  astronomer  sought 
to  secure  priority  for  his  discovery  without  making  it 
known,  by  concealing  it  in  an  anagram,  a  collection  of 
letters  which,  when  properly  arranged,  would  inform  the 


ASPECTS    OF    SATURN'S    RINGS        215 

reader  that  the  companion  of  Saturn  made  its  revolu- 
tion in  fifteen  days.  A  copy  of  this  was  sent  to  Wallis, 
the  celebrated  English  mathematician.  In  his  reply  the 
latter  thanked  Huyghens  for  his  attention,  and  said  he 
also  had  something  to  say,  and  gave  a  collection  of  letters 
longer  than  that  of  Huyghens.  When  the  latter  inter- 
preted his  anagram  to  Wallis,  he  was  surprised  to  receive 
in  reply  a  solution  of  the  Wallis  anagram  announcing 
the  very  same  discovery,  but,  of  course,  in  different  lan- 
guage and  at  greater  length.  It  turned  out  that  Wallis, 
who  was  expert  in  ciphers,  wanted  to  demonstrate  the 
futility  of  the  system,  and  had  managed  to  arrange  his 
own  letters  so  as  to  express  the  discovery,  after  he  knew 
what  it  was.  Huyghens  did  not  appreciate  the  joke. 

Varying  Aspects  of  Saturn 's  Rings 

The  Paris  Observatory  was  founded  in  1666  as  one  of 
the  great  scientific  institutions  of  France  which  adorned 
the  reign  of  Louis  XIV.  Here  Cassini  discovered  the 
division  in  the  ring,  showing  that  the  latter  was  really 
composed  of  two,  one  outside  the  other,  but  in  the  same 
plane.  The  outer  of  these  rings  has  somewhat  the  ap- 
pearance of  being  again  divided,  by  a  line  called  the 
Encke  division,  after  the  astronomer  who  first  noticed  it, 
but  the  exact  nature  of  this  division  is  still  in  doubt.  It 
certainly  is  not  sharp  and  well  defined  like  the  Cassini 
division,  but  only  a  sh'ght  shade. 

To  understand  the  varying  appearance  of  the  rings 
we  give  a  figure  showing  how  they  and  the  planet  would 
look  if  we  could  see  them  perpendicularly  (which  we 


216    PLANETS  AND  THEIR  SATELLITES 


never  can).  We  notice  first  the  dark  Cassini  division, 
separating  the  rings  into  two,  an  inner  and  an  outer  one, 
the  latter  being  the  narrower.  Then,  on  the  outer  ring, 
we  see  the  faint  and  grey  Encke  division,  which  is  much 

less  marked  and  much 
harder  to  see  than 
the  other.  Passing 
to  the  inner  ring, 
the  latter  shades  off 
gradually  on  the  in- 
ner edge,  where  there 
is  a  grey  border 
called  the  "crape 
ring."  This  was  first 
described  by  Bond, 
of  the  Harvard  Ob- 
servatory, and  was 
long  supposed  to  be 
a  separate  and  dis- 
tinct ring.  But  careful  observation  shows  that  such  is 
not  the  case.  The  crape  ring  joins  on  to  the  ring  out- 
side of  it,  and  the  latter  merely  fades  away  into  the 
other. 

The  rings  of  Saturn  are  inclined  about  twenty-seven 
degrees  to  the  plane  of  its  orbit,  and  they  keep  the  same 
direction  in  space  as  the  planet  revolves  round  the  sun. 
The  effect  of  this  will  be  seen  by  the  figure,  which  shows 
the  orbit  of  the  planet  round  the  sun  in  perspective. 
When  the  planet  is  at  A  the  sun  shines  on  the  north 
(upper)  side  of  the  ring.  Seven  years  later,  when  the 


FIG.  39. — Perpendicular  View  of  the  Rings 
of  Saturn. 


ASPECTS    OF    SATURN'S    RINGS        217 

planet  is  at  B,  the  ring  is  presented  to  the  sun  edgewise. 
After  passing  B  the  sun  shines  on  the  south  (lower)  side 
at  an  inclination  which  continually  increases  till  the 
planet  makes  C,  when  the  inclination  is  at  its  greatest, 


. 


£ 

c\ 


FIG.  40. — Showing  how  the  Direction  of  t/ie  Plane  of  Saturn's  Rings  re- 
mains Unchanged  as  the  Planet  moves  round  the  Sun. 

twenty-seven  degrees.  Then  it  diminishes  as  the  planet 
passes  to  D,  at  which  point  the  edge  of  the  ring  is  again 
presented  to  the  sun.  From  this  point  to  A  and  B  the 
sun  again  shines  on  the  north  side. 

The  earth  is  so  near  the  sun  in  comparison  with  Saturn 
that  the  rings  appear  to  us  nearly  as  they  would  to  an 
observer  on  the  sun.  There  is  a  period  of  fifteen  years, 
during  which  we  see  the  north  side  of  the  rings,  and  at 
the  middle  of  which  we  see  them  at  the  widest  angle.  As 
the  years  advance,  the  angle  grows  narrower  and  the 
rings  are  seen  more  and  more  edgewise  till  they  close  up 
into  a  mere  line  crossing  the  planet,  or  perhaps  disappear 
entirely.  Then  they  open  out  again,  to  close  up  in 
another  fifteen  years.  A  disappearance  occurred  in  1892 
and  another  will  take  place  in  1907. 


218    PLANETS  AND  THEIR  SATELLITES 

With  this  view  of  what  the  shape  of  the  rings  really  is, 
we  may  understand  their  appearance  to  us.  The  rings 
are  always  seen  very  obliquely,  never  at  a  greater  angle 
than  twenty-seven  degrees.  The  general  outline  pre- 


FIGS.  41-42. — Disappearance  of  the  Rings  of  Saturn,  according  to  Bar~ 
nard,  when  seen  edgewise. 

sented  by  the  planet  and  rings  is  that  seen  in  Figure  40. 
The  best  views  are  obtained  when  the  rings  are  seen  at  a 
considerable  angle.  The  divisions  and  the  crape  ring 
are  then  seen.  The  shadow  of  the  globe  of  the  planet  on 
the  ring  will  be  seen  as  a  dark  notch.  A  dark  line  cross- 


WHAT    THE    RINGS    ARE  219 

ing  the  planet  like  a  border  to  the  inner  ring  is  the 
shadow  of  the  ring  on  the  planet. 

Very  interesting  are  rather  rare  occasions  when  the 
plane  of  the  ring  passes  between  the  earth  and  the  sun. 
Then  the  sun  shines  on  one  side  of  the  ring  while  the 
other  side  is  presented  to  us,  though,  of  course,  at  a  very 
small  angle.  The  chances  for  observing  Saturn  at  such 
times  are  rather  few,  especially  in  recent  times.  At  both 
the  last  occasions,  1877  and  1892,  this  only  happened 
for  a  few  days,  when  the  planet  was  not  well  situated  for 
these  observations.  Nevertheless,  in  October,  1892,  Bar- 
nard got  a  look  at  it  from  the  Lick  Observatory,  and 
found  that  the  rings  were  totally  invisible,  though  their 
shadow  could  be  seen  on  the  planet.  This  shows  that  the 
rings  are  so  thin  that  their  edges  are  invisible  in  a 
powerful  telescope. 

What  the  Rings  are 

When  it  became  accepted  that  the  laws  of  mechanics, 
as  we  learn  them  on  the  earth,  govern  the  motions  of  the 
heavenly  bodies,  another  riddle  was  presented  by  the 
rings  of  Saturn.  What  keeps  the  rings  in  place  ?  What 
keeps  the  planet  from  running  against  the  inner  ring 
and  producing,  to  modify  Addison's  verse,  a  "wreck  of 
matter  and  crash  of  worlds"  that  would  lay  the  whole 
beautiful  structure  in  ruins  ?  It  was  for  a  time  supposed 
that  a  liquid  ring  might  be  proof  against  such  a  catas- 
trophe, and  then  it  was  shown  that  such  was  not  the  case. 
Finally  it  was  made  clear  that  the  rings  could  not  be  co- 
hering bodies  of  any  kind,  but  were  merely  clouds  of 


PLANETS  AND  THEIR  SATELLITES 

minute  bodies,  perhaps  little  satellites,  perhaps  only  par- 
ticles like  pebbles  or  dust,  or  perhaps  like  a  cloud  of 
smoke.  This  view  had  to  be  accepted,  but  was  long  with- 
out direct  proof.  The  latter  was  finally  brought  out  by 
Keeler  with  his  spectroscope.  He  found  that  when  the 
light  of  the  rings  was  spread  out  into  a  spectrum,  the 
dark  spectral  lines  did  not  go  straight  across  it,  but  were 
bent  and  broken  in  such  a  way  as  to  show  that  the  matter 
of  the  rings  was  revolving  round  the  planet  at  unequal 
speeds.  At  the  outer  edge  it  revolved  most  slowly ;  the 
speed  continually  increased  toward  the  inner  edge,  and 
was  everywhere  the  same  that  a  satellite  would  have  if  it 
revolved  round  the  planet  at  that  distance.  This  most 
beautiful  discovery  was  made  at  the  Allegheny  Observa- 
tory near  Pittsburg,  Pa. 

I 
System  of  Saturn9 s  Satellites 

In  making  known  his  discovery  of  the  satellite  Titan, 
Huyghens  congratulated  himself  that  the  solar  system 
was  now  complete.  There  were  now  seven  great  bodies 
and  seven  small  ones,  the  magic  number  of  each.  But 
within  the  next  thirty  years  Cassini  exploded  all  this 
mysticism  by  discovering  four  more  satellites.  Then, 
after  the  lapse  of  a  century,  the  great  Herschel  found 
yet  two  more.  Finally,  the  eighth  was  found  by  Bond 
at  the  Harvard  Observatory  in  1848. 

In  1898  photographs  of  the  sky  taken  at  the  South 
American  branch  of  the  Harvard  Observatory  showed  a 
star  near  Saturn,  but  farther  than  the  outermost  known 
satellite,  which  seemed  to  be  in  a  different  position  each 


SATELLITES    OF    SATURN 

night.  It  has  not  yet  been  decided  whether  this  was  a 
satellite,  because  Saturn  has  been  among  the  countless 
faint  stars  of  the  Milky  Way,  among  which  the  satellite 
might  be  lost. 

The  following  is  a  list  of  the  eight  satellites,  with 
their  distances  from  the  planet  in  radii  of  the  latter, 
their  times  of  revolution,  and  the  discoverer  of  each : 


No. 

Name. 

Discoverer. 

Date 
of  Dis- 
covery. 

Distance 
from 
Planet. 

Time 
of  Revo- 
lution. 

1 

Mimas  .  .  . 

Herschel    .... 

1789 

3  3 

d.    h. 
0    23 

9, 

Enceledas  . 

Herschel  

1789 

4  3 

1      9 

8 

Tethys 

1684 

5  3 

1    21 

4 

Cassini  

1684 

6  8 

2    18 

5 

Rhea 

Cassini 

1672 

95 

4    12 

6 

Titan 

Huysrhens  ... 

1655 

21  7 

15    23 

7 

Hyperion 

Bond  

1848 

26  8 

21      7 

8 

Tapetus  .  . 

Cassini.-  

1671 

644 

70    22 

The  most  noteworthy  features  of  this  list  are  the  wide 
range  of  distances  among  the  satellites,  and  the  relation 
between  the  times  of  revolution  of  the  four  inner  ones. 
The  five  inner  ones  seem  to  form  a  group  by  themselves. 
Then  there  is  a  gap  exceeding  in  breadth  the  distance  of 
the  innermost  of  the  five,  when  we  have  another  group  of 
two,  Titan  and  Hyperion.  Then  there,  is  a  gap  wider 
than  the  distance  of  Hyperion,  outside  of  which  comes 
Japetus,  the  outermost  yet  known. 

A  curious  relation  among  the  periods  is  that  the  period 
of  the  third  satellite  is  almost  exactly  twice  that  of  the 
first;  and  that  of  the  fourth  almost  twice  that  of  the 


PLANETS  AND  THEIR  SATELLITES 

second.    Also,  four  periods  of  Titan  are  almost  exactly 
equal  to  three  of  Hyperion. 

The  result  of  the  latter  relation  is  a  certain  very  curi- 
ous action  of  these  two  satellites  on  each  other,  through 
their  mutual  gravitation.  To  show  this  we  give  a  dia- 
gram of  the  orbits.  That  of  Hyperion,  the  outer  of  the 


FIG.  43. —  Orbits  of  Titan  and  Hyperion,  showing  their  relation, 

two,  is  very  eccentric,  as  will  be  seen  by  the  figure.  Sup- 
pose the  satellites  to  be  in  conjunction  at  a  certain  mo- 
ment ;  Titan,  the  inner  and  larger  of  the  two  at  a  point 
A,  Hyperion  at  the  point  a  just  outside.  At  the  end  of 
sixty-five  days  Titan  will  have  made  three  revolutions 
and  Hyperion  four,  which  will  bring  them  again  into 


SATELLITES    OF    SATURN  223 

conjunction  at  very  nearly,  but  not  exactly,  the  same 
point.  Titan  will  have  reached  the  point  B,  and  Hy- 
perion b.  At  a  third  conjunction  the  two  will  be  a  little 
above  the  line  Bb,  and  so  on.  Really  the  conjunctions 
occur  closer  together  than  we  have  been  able  to  draw 
them  in  the  figure.  In  the  course  of  nineteen  years  the 
point  of  conjunction  will  have  slowly  moved  all  round 
the  circle,  and  the  satellites  will  again  be  in  conjunction 
at  A. 

Now  the  effect  of  this  slow  motion  of  the  conjunction- 
point  round  the  circle  is  that  the  orbit  of  Hyperion,  or, 
more  exactly,  its  longer  axis,  is  carried  round  with  the 
conjunction-point,  so  that  the  conjunctions  always  occur 
where  the  distance  of  the  two  orbits  is  greatest.  The 
dotted  line  shows  how  the  orbit  of  Hyperion  is  thus  car- 
ried halfway  round  in  nine  years. 

An  interesting  feature  of  this  action  is  that  it  Is,  so 
far  as  we  know,  unique,  there  being  no  case  like  it  else- 
where in  the  solar  system.  But  there  may  be  something 
quite  similar  in  the  mutual  action  of  the  first  and  third, 
and  of  the  second  and  fourth  satellites  of  Saturn  on  each 
other. 

A  yet  more  striking  effect  of  the  mutual  attraction  of 
the  matter  composing  the  rings  and  satellites  is  that, 
excepting  the  outer  satellite  of  all,  these  bodies  all  keep 
exactly  in  the  same  plane.  The  effect  of  the  sun's  at- 
traction, if  there  were  nothing  to  counteract  it,  would 
be  that  in  a  few  thousand  years  the  orbits  of  these  bodies 
would  be  drawn  around  into  different  planes,  all  having, 
however,  the  same  inclination  to  the  plane  of  the  orbit 


PLANETS  AND  THEIR  SATELLITES 

of  Saturn.  But,  by  their  mutual  attraction,  the  planes 
of  the  orbits  are  all  kept  together  as  if  they  were  solidly 
attached  to  the  planet. 

Physical  Constitution  of  Saturn 

There  is  a  remarkable  resemblance  between  the  phys- 
ical make-up  of  this  planet  and  that  of  its  neighbour 
Jupiter.  They  are  alike  remarkable  for  their  small  den- 
sity, that  of  Saturn  being  even  less  than  that  of  water. 
Another  point  of  likeness  is  the  rapid  rotation,  Saturn 
turning  on  its  axis  in  10  hours  14  minutes,  a  little  more 
than  the  period  of  Jupiter.  The  surface  of  the  planet 
also  seems  to  be  variegated  with  cloud-like  forms,  similar 
to  those  of  Jupiter,  but  far  fainter,  so  that  they  cannot 
be  seen  with  any  distinctness. 

What  has  been  said  of  the  probable  cause  of  the  small 
density  of  Jupiter  applies  equally  to  Saturn.  The  prob- 
ability is  that  the  planet  has  a  comparatively  small  but 
massive  nucleus,  surrounded  by  an  immense  atmosphere, 
and  that  what  we  see  is  only  the  outer  surface  of  the 
atmosphere. 

A  curious  fact  which  bears  on  this  view  is  that  the 
satellite  Titan  is  far  denser  than  the  planet.  Its  cubical 
contents  are  about  one  ten-thousandth  those  of  the  planet. 
But  its  mass,  as  found  from  the  motion  of  Hyperion,  is 
one  forty-three-hundredth  that  of  the  planet. 


vm 

URANUS  AND  ITS  SATELLITES 

URANUS  is  the  seventh  of  the  major  planets  in  the  order 
of  distance  from  the  sun.  It  is  commonly  considered  a 
telescopic  planet;  but  one  having  good  eyesight  can 
easily  see  Uranus  without  artificial  help,  if  he  only  knows 
exactly  where  to  look  for  it,  so  as  to  distinguish  it  from 
the  numerous  small  stars  having  the  same  appearance. 
Had  any  of  the  ancient  astronomers  made  so  thorough  an 
examination  of  the  sky  from  night  to  night  as  Dr.  Gould 
did  of  the  southern  heavens  after  he  founded  the  Cordoba 
Observatory,  they  would  have  upset  the  notion  that  there 
were  only  seven  planets. 

Uranus  was  discovered  in  1782  by  Sir  William  Her- 
schel,  who  at  first  supposed  it  to  be  the  nucleus  of  a 
comet.  But  its  motion  soon  showed  that  this  could  not  be 
the  case,  and  before  long  the  discoverer  found  that  it 
was  a  new  addition  to  the  solar  system.  In  gratitude  to 
his  royal  benefactor,  George  III,  he  proposed  to  call  the 
planet  Georgium  Sidus,  a  name  which  was  continued  in 
England  for  some  seventy  years.  Some  continental  as- 
tronomers proposed  that  it  should  be  called  after  its 
discoverer,  and  the  name  Herschel  was  often  assigned  to 
it.  But  by  1850  the  name  Uranus,  originally  proposed 
by  Bode  (author  of  the  "Law"),  and  always  used  in 
Germany,  became  universal. 


226    PLANETS  AND  THEIR  SATELLITES 

When  the  orbit  of  the  planet  was  determined,  so  that 
its  course  in  former  years  could  be  mapped  out,  the  curi- 
ous fact  was  brought  to  light  that  it  had  been  seen  and 
recorded  nearly  a  century  before,  as  well  as  a  few  years 
previously.  Flamsteed,  Astronomer  Royal  of  England, 
while  engaged  in  cataloguing  the  stars,  had  marked  it 
down  as  a  star  on  five  occasions  between  1690  and  1715. 
What  was  yet  more  singular,  Lemonnier,  at  the  Paris  Ob- 
servatory, had  recorded  it  eight  times  in  the  course  of 
two  months,  December,  1768,  and  January,  1769.  But 
he  had  never  reduced  and  compared  his  observations,  and 
not  till  Herschel  announced  the  planet  did  Lemonnier 
know  how  great  a  prize  had  lain  for  ten  years  within 
his  grasp. 

The  period  of  revolution  of  Uranus  is  eighty-four 
years,  so  that  its  position  in  the  sky  changes  but  slowly 
from  year  to  year.  During  the  first  ten  years  of  our  cen- 
tury it  will  be  in  or  near  the  region  of  the  Milky  Way, 
which  we  see  in  summer  and  autumn,  low  down  in  the 
southern  sky.  This  will  make  it  difficult  of  detection  by 
the  naked  eye. 

The  distance  of  Uranus  is  about  twice  that  of  Saturn. 
In  astronomical  units  it  is  19.2 ;  in  our  familiar  measures 
1,790,000,000  miles,  or  2,870,000,000  kilometres. 

Owing  to  this  great  distance,  it  is  hard  to  see  with  cer- 
tainty any  features  on  its  surface.  In  a  good  telescope 
it  appears  as  a  pale  disk  with  a  greenish  hue.  Some  ob- 
servers have  fancied  that  they  saw  faintly  marked  fea- 
tures on  its  surface,  but  this  is  probably  an  illusion.  We 
may  regard  it  as  certain  that  it  rotates  on  its  axis ;  but 


THE    SATELLITES    OF    URANUS        227 

no  ocular  evidence  of  this  has  ever  been  obtained,  and  of 
course  the  period  is  unknown.  But  the  measures  of  Bar- 
nard showed  a  slight  ellipticity  of  the  disk  which,  if  real, 
would  prove  a  rapid  rotation. 

The  spectroscope  shows  that  the  constitution  of 
Uranus  is  materially  different  from  that  of  any  of  the 
six  planets  which  revolve  between  it  and  the  sun.  None 
of  the  latter  gives  a  spectrum  which  is  strikingly  different 
from  that  of  ordinary  sunlight.  But  when  the  light  of 
Uranus  is  spread  out  into  a  spectrum,  a  number  of  more 
or  less  shaded  bands  are  seen,  totally  unlike  the  lines  of 
an  ordinary  spectrum.  Whether  these  bands  are  really 
what  they  appear,  or  whether  they  are  composed  of  a 
multitude  of  fine  dark  lines  invisible  singly,  owing  to 
the  faintness  of  the  light,  has  not  yet  been  ascertained; 
but  the  probabilities  are  that  such  is  the  case.  Whether 
it  is  or  not,  the  spectrum  indicates  that  the  light  reflected 
from  the  planet  has  passed  through  a  dense  medium  of  a 
constitution  quite  different  from  that  of  our  atmosphere. 
But  it  is  as  yet  impossible  to  determine  the  nature  of  this 
medium. 

The  Satellites  of  Uranus 

There  are  four  of  these  bodies  moving  round  Uranus 
as  he  travels  in  his  orbit.  The  two  outer  ones  can  be 
seen  in  a  telescope  of  twelve  inches  aperture  or  more ;  the 
inner  ones  only  in  the  most  powerful  telescopes  of  the 
world.  The  difficulty  of  seeing  them  does  not  arise  from 
their  small  size,  for  they  are  probably  nearly  or  quite  as 
large  as  the  others,  but  from  their  being  blotted  out  by 
the  glare  of  the  planet. 


228    PLANETS  AND  THEIR  SATELLITES 

The  history  of  these  bodies  is  somewhat  peculiar.  Be- 
sides the  two  brighter  ones,  Herschel,  before  1800, 
thought  he  caught  glimpses  from  time  to  time  of  four 
others,  and  thus  it  happened  that  for  more  than  half  a 
century  Uranus  was  credited  with  six  satellites.  This 
was  because  during  all  that  time  no  telescope  was  made 
which  could  claim  superiority  over  Herschel's. 

Then  about  1845,  Lassell,  of  England,  undertook  the 
making  of  reflecting  telescopes,  and  produced  his  two 
great  instruments,  one  of  two,  the  other  of  four  feet 
aperture.  The  latter  he  afterwards  took  to  the  Island 
of  Malta,  in  order  to  make  observations  under  the  fine 
sky  of  the  Mediterranean.  Here  he  and  his  assistant 
entered  upon  a  careful  examination  of  Uranus,  and 
reached  the  conclusion  that  none  of  the  additional  satel- 
lites supposed  by  Herschel  had  any  existence.  But,  on 
the  other  hand,  two  new  ones  were  found  so  near  the 
planet  that  they  could  not  have  been  seen  by  any  pre- 
vious observer.  During  the  next  twenty  years  these 
newly  found  bodies  were  looked  for  in  vain  with  the  best 
telescopes  then  in  use  in  Europe,  and  some  astronomers 
professed  to  doubt  their  existence.  But  in  the  winter  of 
1873  they  were  found  with  the  twenty-six-inch  Wash- 
ington telescope,  which  had  just  been  completed,  and 
were  shown  to  move  in  exact  accordance  with  the  observa- 
tions of  Lassell. 

The  most  remarkable  feature  of  these  bodies  is  that 
their  orbits  are  nearly  perpendicular  to  the  orbit  of  the 
planet.  The  result  is  that  there  are  two  opposite  points 
of  the  latter  orbit  where  that  of  the  satellite  is  seen  edge- 


THE    SATELLITES    OF    URANUS       229 

wise.  When  Uranus  is  near  either  of  these  points,  we, 
from  the  earth,  see  the  satellites  moving  as  if  swinging 
up  and  down  in  a  north  and  south  direction  on  each 
side  of  the  planet,  like  the  bob  of  a  pendulum.  Then,  as 
the  planet  moves  on,  the  apparent  orbits  slowly  open  out. 
At  the  end  of  twenty  years  we  see  them  perpendicularly. 
They  then  seem  to  us  almost  circular,  but  appear  to  close 
up  again  year  after  year  as  the  planet  moves  on  its 
course.  The  orbits  were  last  seen  edgewise  in  1882,  and 
will  be  again  so  seen  about  1924.  For  several  years  to 
come  the  orbits  are  seen  from  a  nearly  perpendicular 
standpoint,  which  is  the  most  favourable  condition  for 
observing  the  satellites. 

It  is  quite  possible  that  continued  observations  of  these 
bodies  will  yet  enable  the  astronomer  to  reach  some  con- 
clusion to  the  hitherto  unsolved  problem  of  the  rotation 
of  Uranus  on  its  axis.  In  the  cases  of  Mars,  Jupiter,  and 
Saturn,  the  satellites  revolve  very  nearly  in  the  plane  of 
the  equators  of  the  several  planets  to  which  they  belong. 
If  this  is  true  of  Uranus,  it  would  follow  that  the  equator 
of  the  planet  was  nearly  perpendicular  to  its  orbit,  and 
ihat  its  north  pole,  at  two  opposite  points  in  its  orbit, 
would  point  almost  exactly  to  the  sun.  Such  being  the 
case,  the  seasons  would  be  vastly  more  marked  than  they 
are  on  our  earth.  Only  on  or  near  the  equator  of  Uranus 
would  a  denizen  of  the  planet  see  the  sun  every  day.  If 
he  lived  in  middle  latitudes  there  would  be  a  period  equal 
in  length  to  five  or  ten  of  our  years  during  which  the  sun 
would  never  reach  his  horizon.  Then,  moving  rapidly 
upwards,  it  would  rise  and  set,  giving  him  day  and  night, 


230    PLANETS  AND  THEIR  SATELLITES 

but  in  time  it  would  get  so  far  up  toward  the  north  pole 
that  it  would  never  set  during  a  period  equal  to  that  at 
which  it  never  rose. 

The  fact  that  all  the  satellites  revolve  in  almost  ex- 
actly the  same  plane  gives  some  colour  to  this  view,  but 
does  not  quite  prove  it,  because  it  is  not  impossible  that 
their  planes  are  kept  together  by  their  mutual  action. 
If,  however,  this  is  the  case,  and  if  the  equator  of  Uranus 
does  not  coincide  with  the  orbits,  the  latter  will,  in  the 
course  of  centuries,  undergo  a  change  which  our  succes- 
sors will  be  able  to  determine.  In  this  way  they  will  be 
enabled  to  learn  something  of  the  equator  and  poles  of 
Uranus,  even  if  their  telescopes  are  not  powerful  enough 
to  afford  any  visual  evidence  on  the  subject. 


IX 

NEPTUNE  AND  ITS  SATELLITE 

So  far  as  yet  known,  Neptune  is  the  outermost  planet 
of  our  solar  system.  In  size  and  mass  it  is  not  very 
different  from  Uranus,  but  its  greater  distance,  30  astro- 
nomical units,  instead  of  19.2,  makes  it  fainter  and 
harder  to  see.  It  is  far  below  the  limit  of  visibility  by 
the  naked  eye,  but  quite  a  moderate-sized  telescope  would 
show  it  if  one  could  only  distinguish  it  from  the  numer- 
ous stars  of  similar  brightness  that  stud  the  heavens. 
This  needs  astronomical  appliances  of  a  more  refined 
and  complex  sort. 

The  disk  of  Neptune  is  to  be  made  out  only  with  a 
telescope  of  considerable  power.  It  is  then  seen  to  be  of  a 
bluish  or  leaden  tint,  perceptibly  different  from  the  sea- 
green  of  Uranus.  Of  course  nothing  can  be  known  by 
direct  observation  about  its  rotation  on  its  axis.  Its  spec- 
trum shows  bands  like  those  of  Uranus,  and  it  seems  likely 
that  the  two  bodies  are  much  alike  in  their  constitution. 

The  discovery  of  Neptune  in  1846  is  regarded  as  one 
of  the  most  remarkable  triumphs  of  mathematical  as- 
tronomy. Its  existence  was  made  known  by  its  attraction 
on  the  planet  Uranus  before  any  other  evidence  had  been 
brought  out.  The  history  of  the  circumstances  leading 
to  the  discovery  is  so  interesting  that  we  shall  briefly 
mention  its  main  points. 


PLANETS  AND  THEIR  SATELLITES 


History  of  the  Discovery  of  Neptune 

During  the  first  twenty  years  of  the  nineteenth  cen- 
tury Bouvard,  of  Paris,  an  eminent  mathematical  astron- 
omer, prepared  new  tables  of  the  motions  of  Jupiter, 
Saturn,  and  Uranus,  then  supposed  to  be  the  three  outer- 
most planets.  He  took  the  deviations  of  these  planets, 
produced  by  their  attraction  on  each  other,  from  the 
calculations  of  Laplace.  He  succeeded  fairly  well  in 
fitting  his  tables  to  the  observed  motions  of  Jupiter  and 
Saturn,  but  found  that  all  his  efforts  to  make  tables  that 
would  agree  with  the  observed  positions  of  Uranus  were 
fruitless.  If  he  considered  only  the  observations  made 
since  the  discovery  by  Herschel,  he  could  get  along ;  but 
no  agreement  could  be  obtained  with  those  made  previ- 
ously by  Flamsteed  and  Lemonnier,  when  the  planet  was 
supposed  to  be  a  fixed  star.  So  he  rejected  these  old 
observations,  fitted  his  orbit  into  the  modern  ones,  and 
published  his  tables.  But  it  was  soon  found  that  the 
planet  began  to  move  away  from  its  calculated  position, 
and  astronomers  began  to  wonder  what  was  the  matter. 
It  was  true  that  the  deviation,  measured  by  a  naked  eye 
standard,  was  very  small;  in  fact,  if  there  had  been  two 
planets,  one  in  the  real  and  one  in  the  calculated  position, 
the  naked  eye  could  not  have  distinguished  them  from 
a  single  star.  But  the  telescope  would  have  shown  them 
well  separated. 

Thus  the  case  stood  until  1845.  At  that  time  there 
lived  in  Paris  a  young  mathematical  astronomer,  Lever- 
ner,  who  had  already  made  a  name  in  his  science,  having 


DISCOVERY    OF    NEPTUNE  233 

communicated  to  the  Academy  of  Sciences  some  re- 
searches which  gave  Arago  a  very  high  opinion  of  his 
abilities.  Arago  called  his  attention  to  the  case  of 
Uranus  and  suggested  that  he  should  investigate  the  sub- 
ject. The  idea  occurred  to  Leverrier  that  the  deviations 
were  probably  caused  by  the  attraction  of  an  unknown 
planet  outside  of  Uranus.  He  proceeded  to  calculate  in 
what  orbit  a  planet  should  move  to  produce  them,  and 
laid  his  result  before  the  Academy  of  Sciences  in  the 
summer  of  1846. 

It  happened  that,  before  Leverrier  commenced  his 
work,  an  English  student  at  the  University  of  Cam- 
bridge, Mr.  John  C.  Adams,  had  the  same  idea  and  set 
about  the  same  work.  He  got  the  result  even  before 
Leverrier  did,  and  communicated  it  to  the  Astronomer 
Royal.  Both  computers  calculated  the  present  position 
of  the  unknown  planet,  so  that,  were  it  possible  to  dis- 
tinguish it  from  a  fixed  star,  it  would  only  have  been 
necessary  to  search  in  the  region  indicated  in  order  to 
find  the  planet.  Unfortunately,  however,  Airy  was  in- 
credulous as  to  the  matter,  and  did  not  think  the  chance 
,pf  finding  the  planet  sufficient  to  go  through  the  labori- 
ous operation  of  a  search  until  his  attention  was  attract- 
ed by  the  prediction  of  Leverrier,  and  the  close  agree- 
ment between  the  two  computers  was  remarked. 

The  problem  of  finding  the  planet  was  now  taken  up. 
Very  thorough  observations  were  made  upon  the  stars  in 
the  region  by  Professor  Challis  at  the  Cambridge  Obser- 
vatory. I  must  explain  that,  as  it  was  not  easy  with  the 
imperfect  instruments  of  that  time  to  distinguish  so 


PLANETS  AND  THEIR  SATELLITES 

small  a  planet  from  the  great  number  of  fixed  stars 
which  studded  the  heavens  around  it,  it  was  necessary 
to  proceed  by  determining  the  position  of  as  many  stars 
as  possible  several  times,  in  order  that,  by  a  comparison 
of  the  observations,  it  could  be  determined  whether  any 
of  them  had  moved  out  of  its  place. 

While  Mr.  Challis  was  engaged  in  this  work  it  oc- 
curred to  Leverrier  that  the  astronomers  of  Berlin  were 
mapping  the  heavens.  He  therefore  wrote  to  Encke,  the 
director  of  the  Berlin  Observatory,  suggesting  that  they 
should  look  for  the  planet.  Now  it  happened  that  the 
Berlin  astronomers  had  just  completed  a  map  of  that 
part  of  the  sky  in  which  the  planet  was  located.  So,  on 
the  very  evening  after  the  letter  was  received,  they  took 
the  map  to  the  telescope  and  proceeded  to  search  about 
to  see  if  any  object  was  seen  in  the  telescope  which  was 
not  on  the  map.  Such  an  object  was  very  soon  found, 
and,  by  comparing  its  position  with  that  of  the  stars 
around  it,  it  seemed  to  have  a  slight  motion.  But  Encke 
was  very  cautious  and  waited  for  the  discovery  to  be  con- 
firmed on  the  night  following.  Then  it  was  found  to 
have  moved  so  much  that  no  doubt  could  remain,  and  he 
wrote  Leverrier  that  the  planet  actually  existed. 

When  this  news  reached  England,  Professor  Challis 
proceeded  to  examine  his  own  observations,  and  found 
that  he  had  actually  observed  the  planet  on  two  occa- 
sions. Unfortunately,  however,  he  had  not  reduced  and 
compared  his  observations,  and  so  failed  to  recognise  the 
object  until  after  it  had  been  seen  at  Berlin. 

The  question  of  the  credit  due  to  Adams  gave  rise  to 


THE    SATELLITE    OF    NEPTUNE       235 

much  controversy,  Arago  in  France  claiming  that,  in  the 
history  of  the  affair,  the  name  of  Adams  should  not  even 
be  mentioned — the  whole  credit  should  go  to  Leverrier. 
This  he  did  on  the  principle  that  it  was  not  the  person 
who  first  did  a  thing,  but  he  who  first  published  it,  who 
should  receive  the  credit.  But  the  English  claimed  that, 
as  Adams  had  actually  preceded  Leverrier  and,  if  he  had 
not  printed  his  paper,  had  at  least  communicated  it  to 
public  authorities,  and  had  enabled  Challis  to  see,  al- 
though not  to  recognise,  the  planet,  he  should  get  his  due 
share  of  credit.  The  whole  question  thus  raised  was  one 
of  honour,  and  subsequent  astronomers  have  taken  the 
very  proper  course  of  honouring  both  men  all  they  could 
for  so  wonderful  a  work. 

The  Satellite  of  Neptune 

Of  course  the  newly  found  planet  was  observed  by 
astronomers  the  world  over.  The  result  was  that  Mr. 
Lassell  soon  found  that  Neptune  was  accompanied  by 
a  satellite.  This  object  was  observed  at  the  few  observa- 
tories then  possessing  telescopes  of  sufficient  power  to 
"make  it  out.  Its  time  of  revolution  was  found  to  be 
nearly  six  days. 

The  most  curious  feature  of  this  satellite  is  that,  con- 
trary to  the  rule  in  the  case  of  all  the  bodies  of  the  solar 
system  except  Uranus,  it  moves  from  east  toward  west. 
In  the  case  of  Uranus  we  cannot  consider  the  motion  as 
being  east  or  west,  we  should  rather  call  it  a  north  and 
south  motion. 

It  would  be  very  interesting  to  know  whether  the 


236    PLANETS  AND  THEIR  SATELLITES 

planet  Neptune  revolves  on  its  axis  in  the  same  direction 
as  the  satellite  moves.  But  this  cannot  be  determined, 
because  it  is  so  distant  and  its  disk  so  faint  and  diffuse 
that  no  markings  can  be  detected  upon  it.  Indeed,  if  we 
reflect  that  the  rotation  of  a  planet  so  near  us  as  Venus 
has  never  been  certainly  determined,  we  may  easily  see 
how  hopeless  is  the  prospect  of  determining  that  of 
Neptune. 

But,  in  spite  of  this,  there  is  remarkable  evidence  that 
the  planet  has  a  rapid  rotation.  It  is  found  that  the 
orbit  of  the  satellite  is  very  slowly  changing  its  position 
from  year  to  year.  During  the  half  century  since  obser- 
vations commenced,  this  change  amounts  to  several  de- 
grees. The  only  way  in  which  it  can  be  accounted  for 
is  by  supposing  that  Neptune,  like  the  earth  and  the  other 
rapidly  rotating  planets,  is  an  oblate  ellipsoid,  and  that 
the  plane  of  the  planet's  equator  does  not  coincide  with 
that  of  the  orbit  of  the  satellite.  In  time  the  astronomer 
will  be  able  to  learn  from  this  motion  the  position  of  the 
poles  and  equator  of  the  planet  Neptune,  but  this  may 
require  a  century  of  observation,  or  even  several  centuries. 


X 

How  THE  HEAVENS  ARE  MEASURED 

DISTANCES  in  the  heavens  may  be  determined  by  a 
method  similar  to  that  employed  by  an  engineer  in  de- 
termining the  distance  of  an  inaccessible  object — -say  a 
mountain  peak.  Two  points,  A  and  B,  are  taken  as  a 
base  line  from  which  to  measure  the  distance  of  a  third 
point,  C.  Setting  up  his  instrument  at  A,  the  engineer 
measures  the  angle  between  B  and  C.  Setting  it  up  at 
B  he  measures  the  angle  between  A  and  C.  Since  the 
sum  of  the  three  angles  of  a  triangle  is  always  one  hun- 
dred and  eighty  degrees,  the  angle  at  C  is  found  by 

.———«.— —•-—— ————«— —--——-———_—-- — __•_  __,___..„, 


FIG.  44. — Measure  of  the  Distance  of  an  Inaccessible  Object  by  Triangulation. 

subtracting  the  sum  of  the  angles  at  A  and  B  from  that 
quantity.  It  will  readily  be  seen  that  the  angle  at  C 
is  that  subtended  by  the  base  line  as  it  would  appear  if 
viewed  by  an  observer  at  C.  Such  an  angle  is,  in  a 
general  way,  called  a  parallax.  It  is  the  difference  of 
direction  of  the  point  C  as  seen  from  the  points  A  and  B. 
It  will  readily  be  seen  that,  with  a  given  base  line, 


238    PLANETS  AND  THEIR  SATELLITES 

the  greater  the  distance  of  the  object  the  less  will  be  its 
parallax.  At  a  sufficiently  great  distance  the  latter  will 
be  so  small  that  the  observer  cannot  get  any  evidence  of 
it.  To  all  appearance  the  lines  B  C  and  A  C  will  then 
have  the  same  direction.  The  distance  at  which  the 
parallax  cannot  be  made  out  depends,  of  course,  on  the 
accuracy  of  the  measurement,  and  the  length  of  the 
base  line. 

The  moon  being  the  nearest  of  all  the  heavenly  bodies 
has  the  largest  parallax.  Its  distance  can  therefore  be 
determined  with  the  greatest  precision  by  measurement. 
Even  Ptolemy,  who  lived  only  one  or  two  centuries  after 
Christ,  was  able  to  make  an  approximate  measure  of  the 
distance  of  the  moon.  But  the  parallax  of  a  planet  is  so 
small  that  it  can  be  determined  only  with  the  most  refined 
instruments. 

The  ends  of  the  base  line  used  in  the  determination 
may  be  any  two  points  on  the  earth's  surface — say  the 
observatories  of  Greenwich  and  the  Cape  of  Good  Hope. 
In  the  case  of  the  transits  of  Venus,  which  we  have  al- 
ready described,  there  were  a  number  of  different  sta- 
tions at  various  points  on  the  earth's  surface,  from  which 
the  direction  of  Venus  at  the  beginning  and  end  of  its 
transit  could  be  inferred.  This  method  of  determining 
distances  is  called  triangulation. 

The  idea  of  a  triangulation,  as  thus  set  forth,  gives  an 
understanding  only  of  the  general  principle  involved  in 
the  problem.  One  can  readily  see  that  it  would  be  out 
of  the  question  for  two  observers  in  distant  parts  of  the 
earth  to  get  the  exact  direction  of  a  planet  at  the  same 


THE    MOTION    OF    LIGHT  239 

moment  of  time.  The  actual  determination  of  the  paral- 
lax requires  a  combination  of  observations  too  complex 
to  be  set  forth  in  the  present  book,  but  the  fundamental 
principle  is  that  just  explained. 

In  order  to  get  the  dimensions  of  the  whole  solar  sys- 
tem, it  is  only  necessary  to  know  the  distance  of  any  one 
planet  from  us  at  any  given  moment.  The  orbits  and 
motions  of  all  the  planets  are  mapped  down  with  the 
greatest  possible  exactness,  but  with  the  map  before  us 
we  are  in  the  position  that  one  would  be  who  had  a  very 
exact  map  of  a  country,  only  there  was  no  scale  of  miles 
upon  it.  So  he  would  be  unable  to  measure  the  distance 
from  one  point  to  another  on  his  map  until  he  knew  the 
scale.  It  is  the  scale  of  our  map  of  the  solar  system 
which  the  astronomer  stands  in  need  of  and  which  he  has 
not,  even  with  the  most  refined  instruments,  yet  been  able 
to  determine  as  accurately  as  he  could  wish. 

The  fundamental  unit  aimed  at  is  that  already  de- 
scribed— the  mean  distance  of  the  earth  from  the  sun. 
Measures  of  parallax  are  by  no  means  the  only  method  of 
determining  this  distance.  Within  the  last  fifty  years 
other  methods  have  been  developed,  some  of  which  are 
fully  as  accurate  as  the  best  measures  of  parallax,  per- 
haps even  more  so. 

Measurement  by  the  Motion  of  Light 

One  of  the  most  simple  and  striking  of  these  methods 
makes  use  of  the  velocity  of  light.  By  observations  of 
Jupiter's  satellites,  made  when  the  earth  was  at  different 
points  of  its  orbit,  it  has  been  found  that  light  passes 


240    PLANETS  AND  THEIR  SATELLITES 

over  a  distance  equal  to  that  of  the  earth  from  the  sun 
in  about  eight  minutes  twenty  seconds,  or  five  hundred 
seconds.  This  determination  has  been  more  accurately 
made  in  another  way  by  the  aberration  of  the  stars. 
This  is  a  slight  change  in  their  position  due  to  the  com- 
bined motion  of  the  earth  and  the  ray  of  light  by  which 
we  see  the  star.  By  accurate  observations  on  the  aberra- 
tion, it  is  found  that  light  travels  from  the  earth  to  the 
sun  in  almost  exactly  499.6  seconds.  It  follows  that  if  we 
can  find  how  far  light  will  travel  in  one  second,  we  can 
determine  the  distance  of  the  sun  by  multiplying  the  re- 
sult by  499.6.  The  measurement  of  the  velocity  of  light 
is  one  of  the  most  difficult  problems  in  physics,  as  it  re- 
quires the  measurement  of  intervals  of  time  only  a  few 
millionths  of  a  second  in  duration.  Those  who  are  inter- 
ested in  the  subject  will  see  the  method  of  doing  this 
explained  in  special  treatises ;  at  present  it  is  sufficient  to 
say  that  light  is  found  to  travel  299,860  kilometres,  or 
186,300  miles  in  a  second.  Multiply  this  by  499-6  and 
we  have  93,075,480  miles  for  the  distance  of  the  sun  from 
the  earth. 

Measurement  by  the  Sun's  Gravitation 

A  third  method  rests  on  the  measures  of  the  sun's 
gravitation  upon  the  moon.  One  effect  of  this  is  that, 
as  the  moon  performs  its  monthly  revolution  round  the 
earth,  it  is  at  its  first  quarter  a  little  more  than  two 
minutes  behind  its  average  position,  to  which  it  catches 
up  at  full  moon,  and  passes ;  so  that  at  last  quarter  it  is 
two  minutes  ahead  of  the  mean  position.  Toward  new 


THE    SUN'S    GRAVITATION  2U 

moon  it  falls  behind  again  to  the  average  place.  Thus 
a  slight  swing  goes  on  in  unison  with  the  moon's  mo- 
tion around  the  earth.  The  amount  of  this  swing  is 
inversely  proportional  to  the  distance  of  the  sun.  Hence, 
by  measuring  this  amount,  its  distance  may  be  deter- 
mined. As  in  other  astronomical  measurements,  the 
difficulty  of  the  determination  is  very  great.  A  swing 
like  this  is  very  hard  to  measure  without  error;  more- 
over, the  problem  of  determining  just  how  much  swing 
the  sun  would  produce  at  a  given  distance  is  one  of  the 
difficult  problems  of  celestial  mechanics,  which  has  not 
yet  been  solved  so  satisfactorily  as  to  leave  no  doubt 
whatever  on  the  result. 

The  fourth  method  also  rests  on  gravitation.  If  we 
only  knew  the  exact  relation  between  the  mass  of  the 
earth  and  that  of  the  sun;  that  is  to  say,  if  we  could 
determine  precisely  how  many  times  heavier  the  sun  is 
than  the  earth,  we  could  compute  at  what  distance  the 
earth  must  be  placed  from  the  sun  in  order  to  revolve 
around  it  in  one  year.  The  only  difficulty,  therefore,  is 
to  weigh  the  earth  against  the  sun.  This  is  most  exactly 
^done  by  finding  the  change  in  the  position  of  the  orbit 
of  Venus  produced  by  the  earth's  attraction.  By  com- 
paring the  positions  of  the  orbit  of  Venus  by  its  transits 
in  1761,  1769,  1874,  and  1882,  it  is  found  that  the  orbit 
has  a  progressive  motion,  indicating  that  the  mass  of 
the  sun  is  332,600  times  that  of  the  earth  and  moon  com- 
bined. Thus  we  are  enabled  to  compute  the  distance  of 
the  sun  by  still  another  method. 


PLANETS  AND  THEIR  SATELLITES 


Results  of  Measurements  of  the  Sun's  Distance 

We  have  described  four  methods  of  making  this  fun- 
damental determination  in  astronomy,  and  in  order  that 
the  reader  may  see  just  what  degree  of  certainty  and 
precision  astronomical  theory  and  measurements  have 
reached,  we  give  the  separate  results  of  these  methods. 
The  first  column  shows  the  parallax  of  the  sun,  which 
is  the  quantity  actually  used  by  astronomers.  It  is  the 
same  thing  as,  the  angle  under  which  the  equatorial 
radius  of  the  earth  would  be  seen  by  an  observer  at  the 
distance  of  the  sun  from  us.  This  is  followed  by  the 
accompanying  distance  in  miles. 

// 

Measures  of  parallax  .....  .  8.800;  Dist.  92,908,000  miles 

Velocity  of  light  ........  ...  8.778;      "     93,075,480      " 

Motion  of  moon  ...........  8.784;      "     92,958,000      " 

Mass  of  the  earth  .........  8.762;      "     93,113,000      " 

The  difference  between  these  results  is  no  greater  than 
the  liability  of  error  wherever  mathematical  demonstra- 
tions and  instrumental  measurements  of  such  extreme 
minuteness  and  complexity  as  these  are  required.  From 
the  close  agreement  between  results  reached  by  methods 
so  widely  different  in  their  principles,  we  have  a  striking 
proof  of  the  correctness  of  the  astronomical  views  of  the 
universe.  Yet  discrepancies  exceeding  a  hundred  thou- 
sand miles  will  not  be  tolerated  by  astronomers  longer 
than  is  absolutely  necessary. 


XI 

GRAVITATION   AND   THE  WEIGHING   OF   THE   PLANETS 

No  work  of  the  human  intellect  farther  transcends 
what  would  seem  possible  to  the  ordinary  thinker  than  the 
mathematical  demonstrations  of  the  motions  of  the  heav- 
enly bodies  under  the  influence  of  their  mutual  gravita- 
tion. We  have  learned  something  of  the  orbits  of  the 
planets  round  the  sun ;  but  the  following  of  the  orbit  is 
not  the  fundamental  law  of  the  planet's  motion ;  the  lat- 
ter is  determined  by  gravitation  alone.  This  law,  as 
stated  by  Newton,  is  so  comprehensive  that  nothing  can 
be  added.  The  law  that  every  particle  of  matter  in  the 
universe  attracts  every  other  particle,  with  a  force  which 
varies  inversely  as  the  square  of  the  distance  between 
them,  is  the  only  law  of  nature  which,  so  far  as  we  know, 
is  absolutely  universal  and  invariable  in  its  action.  All 
the  other  processes  of  nature  are  in  some  way  varied  or 
,  modified  by  heat  and  cold,  by  time  or  place,  by  the  pres- 
ence or  absence  of  other  bodies.  But  no  operation  that 
man  has  ever  been  able  to  perform  on  matter  changes  its 
gravitation  in  the  slightest.  Two  bodies  gravitate  by 
exactly  the  same  amount,  no  matter  what  we  do  with 
them,  no  matter  what  obstacles  we  interpose  between 
them,  no  matter  how  fast  they  move.  All  other  natural 
forces  admit  of  being  investigated,  but  gravitation  does 
not.  Philosophers  have  attempted  to  explain  it,  or  to 


PLANETS  AND  THEIR  SATELLITES 

find  some  cause  for  it,  but  nothing  has  yet  been  added 
to  our  knowledge  by  these  attempts. 

The  motions  of  the  planets  are  governed  by  their 
gravitation.  Were  there  only  a  single  planet  moving 
round  the  sun  it  would  be  acted  on  by  no  force  but  the 
sun's  attraction.  By  purely  mathematical  calculation  it 
is  shown  that  such  a  planet  would  describe  an  ellipse, 
having  the  sun  in  one  focus.  It  would  keep  going  round 
and  round  in  this  ellipse  forever.  But  in  accordance  with 
the  law,  the  planets  must  gravitate  towards  each  other. 
This  mutual  gravitation  is  far  less  than  that  of  the  sun, 
because  in  our  solar  system  the  planets  are  of  much 
smaller  mass  than  the  central  body.  In  consequence  of 
this  mutual  attraction  the  planets  deviate  from  the 
ellipse.  Their  orbits  are  very  nearly,  but  not  exactly, 
ellipses.  Still,  the  problem  of  their  motion  is  one  of  pure 
mathematical  demonstration.  It  has  occupied  the  ablest 
mathematicians  of  the  world  since  the  time  of  Newton. 
Every  generation  has  studied  and  added  to  the  work  of 
the  preceding  one.  One  hundred  years  after  Newton, 
Laplace  and  Lagrange  showed  that  the  ellipses  near 
which  the  planets  move  gradually  change  their  form  and 
position.  These  changes  can  be  calculated  thousands, 
tens  of  thousands,  or  even  hundreds  of  thousands  of 
years  in  advance.  Thus  it  is  known  that  the  eccentricity 
of  the  earth's  orbit  round  the  sun  is  now  slightly 
diminishing,  and  that  it  will  continue  to  diminish  for 
about  forty  thousand  years.  Then  it  will  increase  so 
that  in  the  course  of  many  thousands  more  of  years  it 
will  be  greater  than  it  now  is.  The  same  is  true  of  all 


GRAVITATION    AND    WEIGHING 

the  planets.  Their  orbits  gradually  change  their  form 
back  and  forth  through  tens  of  thousands  of  years,  like 
"great  clocks  of  eternity  which  count  off  ages  as  ours 
count  off  seconds."  The  ordinary  reader  would  be  justi- 
fied in  some  incredulity  as  to  the  correctness  of  these 
predictions  for  thousands  of  years  to  come,  were  it  not 
for  the  striking  precision  with  which  the  motions  of  the 
planets  are  actually  predicted  by  the  mathematical 
astronomer.  This  precision  is  reached  by  solving  the 
very  difficult  problem  of  determining  the  effect  of  each 
planet  on  the  motions  of  all  the  other  planets.  We  might 
predict  the  motions  of  these  bodies  by  assuming  that  each 
of  them  moves  round  the  sun  in  a  fixed  ellipse,  which, 
as  I  have  just  said,  would  be  the  case  if  it  were  not 
attracted  by  any  other  body.  Our  predictions  would 
then,  from  time  to  time,  be  in  error  by  amounts  which 
might  amount  to  large  fractions  of  a  degree;  perhaps, 
in  the  course  of  a  long  time,  to  even  more.  To  form  an 
idea  of  this  error  we  may  say  that  one  degree  is  the  angle 
at  which  we  see  the  breadth  of  an  ordinary  window  frame 
at  the  distance  of  a  hundred  yards.  The  planet  might 
"then  be  predicted  as  in  a  line  with  one  side  of  such  a 
frame  when  in  reality  it  would  be  at  the  other  side  or  in 
the  middle  of  the  window. 

But,  taking  account  of  the  attraction  of  all  the  other 
planets,  the  prediction  is  so  exact  that  the  refined  obser- 
vations of  astronomy  hardly  show  any  appreciable  de- 
viation. If  we  should  mark  on  the  side  of  a  distant  house 
a  row  of  a  hundred  points,  each  apparently  as  far  from 
the  other  as  the  average  error  of  these  predictions,  the 


246    PLANETS  AND  THEIR  SATELLITES 

whole  row  would  seem  to  the  naked  eye  as  a  single  point. 
The  history  of  the  discovery  of  Neptune,  which  was 
mentioned  in  the  preceding  chapter,  affords  the  most 
striking  example  that  we  possess  of  the  certainty  of  these 
predictions. 

How  the  Planets  are  Weighed 

I  shall  now  endeavour  to  give  the  reader  some  idea  of 
the  manner  in  which  the  mathematical  astronomer  reaches 
these  wonderful  results.  To  make  them,  he  must,  of 
course,  know  the  pull  each  planet  exerts  upon  the  others. 
This  is  proportional  to  what  the  physicist  and  astrono- 
mer call  the  mass  of  the  attracting  planet.  This  word 
means  quantity  of  matter,  and  around  us  on  the  surface 
of  the  earth,  it  has  nearly  the  same  meaning  as  the  word 
weight.  We  may  therefore  say  that,  when  the  astrono- 
mer determines  the  mass  of  a  planet,  he  is  weighing  it. 
He  does  this  on  the  same  principle  by  which  the  butcher 
weighs  a  ham  in  the  spring  balance.  When  the  butcher 
picks  the  ham  up  he  feels  a  pull  of  the  ham  toward  the 
earth.  When  he  hangs  it  on  the  hook,  this  pull  is  trans- 
ferred from  his  hand  to  the  spring  of  the  balance.  The 
stronger  the  pull  the  farther  the  spring  is  pulled  down. 
What  he  reads  on  the  scale  is  the  strength  of  the  pull. 
You  know  that  this  pull  is  simply  the  attraction  of  the 
earth  on  the  ham.  But,  by  a  universal  law  of  force,  the 
ham  attracts  the  earth  exactly  as  much  as  the  earth  does 
the  ham.  So  what  the  butcher  really  does  is  to  find  how 
much  or  how  strongly  the  ham  attracts  the  earth,  and 
he  calls  that  pull  the  weight  of  the  ham.  On  the  same 


HOW  THE  PLANETS  ARE  WEIGHED 

principle,  the  astronomer  finds  the  weight  of  a  body  by 
finding  how  strong  is  its  attractive  pull  on  some  other 
body. 

In  applying  this  principle  to  the  heavenly  bodies,  you 
meet  at  once  a  difficulty  that  looks  insurmountable.  You 
cannot  get  up  to  the  heavenly  bodies  to  do  your  weigh- 
ing ;  how  then  will  you  measure  their  pull  ?  I  must  begin 
the  answer  to  this  question  by  explaining  more  exactly 
the  difference  between  the  weight  of  a  body  and  its  mass. 
The  weight  of  objects  is  not  the  same  all  over  the  world; 
a  thing  which  weighs  thirty  pounds  in  New  York  would 
weigh  an  ounce  more  than  thirty  pounds  in  a  spring 
balance  in  Greenland,  and  nearly  an  ounce  less  at  the 
equator.  This  is  because  the  earth  is  not  a  perfect  sphere, 
but  a  little  flattened.  Thus  weight  varies  with  the  place. 
If  a  ham  weighing  thirty  pounds  were  taken  up  to  the 
moon  and  weighed  there,  the  pull  would  only  be  five 
pounds,  because  the  moon  is  so  much  smaller  and  lighter 
than  the  earth.  But  there  would  be  just  as  much  ham 
on  the  moon  as  on  the  earth.  There  would  be  another 
weight  of  the  ham  on  the  planet  Mars,  and  yet  another 
on  the  sun,  where  it  would  weigh  some  eight  hundred 
pounds.  Hence,  the  astronomer  does  not  speak  of  the 
weight  of  a  planet,  because  that  would  depend  on  the 
place  where  it  was  weighed ;  but  he  speaks  of  the  mass  of 
the  planet,  which  means  how  much  planet  there  is,  no 
matter  where  you  might  weigh  it. 

At  the  same  time  we  might,  without  any  inexactness, 
agree  that  the  mass  of  a  heavenly  body  should  be  fixed  by 
the  weight  it  would  have  at  some  place  agreed  upon,  say 


248    PLANETS  AND  THEIR  SATELLITES 

New  York.  As  we  could  not  even  imagine  a  planet  at 
New  York,  because  it  may  be  larger  than  the  earth  itself, 
what  we  are  to  imagine  is  this :  Suppose  the  planet  could 
be  divided  into  a  million  million  million  equal  parts,  and 
one  of  these  parts  brought  to  New  York  and  weighed. 
We  could  easily  find  its  weight  in  pounds  or  tons.  Then 
multiply  this  weight  by  a  million  million  million  and  we 
shall  have  a  weight  of  the  planet.  This  would  be  what 
the  astronomers  might  take  as  the  mass  of  the  planet. 

With  these  explanations,  let  us  see  how  the  weight  of 
the  earth  is  found.  The  principle  we  apply  is  that  round 
bodies  of  the  same  specific  gravity  attract  small  objects 
on  their  surface  with  a  force  proportional  to  the  diameter 
of  the  attracting  body.  For  example,  a  body  two  feet 
in  diameter  attracts  twice  as  strongly  as  one  of  a  foot, 
one  of  three  feet  three  times  as  strongly,  and  so  on.  Now, 
our  earth  is  about  forty  million  feet  in  diameter ;  that  is, 
ten  million  times  four  feet.  It  follows  that  if  we  made 
a  little  model  of  the  earth  four  feet  in  diameter,  having 
the  average  specific  gravity  of  the  earth,  it  would  attract 
a  particle  with  one  ten-millionth  part  of  the  attraction 
of  the  earth.  We  have  shown  in  our  chapter  on  the  earth 
how  the  attraction  of  such  a  model  has  actually  been 
measured,  with  the  result  of  showing  that  the  total  mass 
of  the  earth  is  five  and  one  half  times  that  of  an 
equal  bulk  of  water.  Thus  this  mass  becomes  a  known 
quantity. 

We  come  now  to  the  planets.  I  have  said  that  the 
mass  or  weight  of  a  heavenly  body  is  determined  by  its 
attraction  on  some  other  body.  There  are  two  ways  in 


HOW  THE  PLANETS  ARE  WEIGHED  249 

which  the  attraction  of  a  planet  may  be  measured.  One 
is  by  its  attraction  on  the  planets  next  to  it,  causing 
them  to  deviate  from  the  orbits  in  which  they  would  move 
if  left  to  themselves.  By  measuring  the  deviations,  we 
can  determine  the  amount  of  the  pull,  and  hence  the  mass 
of  the  planet. 

The  reader  will  readily  understand  that  the  mathe- 
matical processes  necessary  to  get  a  result  in  this  way 
must  be  very  delicate  and  complicated.  A  much  simpler 
method  can  be  used  in  the  case  of  those  planets  which 
have  satellites  revolving  round  them,  because  the  attrac- 
tion of  the  planet  can  be  determined  by  the  motions  of 
the  satellite.  The  first  law  of  motion  teaches  us  that  a 
body  in  motion,  if  acted  on  by  no  force,  will  move  in  a 
straight  line.  Hence,  if  we  see  a  body  moving  in  a  curve, 
we  know  that  it  is  acted  on  by  a  force  in  the  direction 
toward  which  the  motion  curves.  A  familiar  example  is 
that  of  a  stone  thrown  from  the  hand.  If  the  stone  were 
not  attracted  by  the  earth  it  would  go  on  forever  in  the 
line  of  throw,  and  leave  the  earth  entirely.  But  under 
the  attraction  of  the  earth  it  is  drawn  down  and  down, 
as  it  travels  onward,  until  finally  it  reaches  the  ground. 
The  faster  the  stone  is  thrown,  of  course,  the  farther  it 
will  go,  and  the  greater  will  be  the  sweep  of  the  curve  of 
its  path.  If  it  were  a  cannon  ball,  the  first  part  of  the 
curve  would  be  nearly  a  right  line.  If  we  could  fire  a 
cannon  ball  horizontally  from  the  top  of  a  high  moun- 
tain with  a  velocity  of  five  miles  a  second,  and  if  it  were 
not  resisted  by  the  air,  the  curvature  of  the  path  would 
be  equal  to  that  of  the  surface  of  our  earth,  and  so  the 


250    PLANETS  AND  THEIR  SATELLITES 

ball  would  never  reach  the  earth,  but  would  revolve  round 
it  like  a  little  satellite  in  an  orbit  of  its  own.  Could  this 
be  done  the  astronomer  would  be  able,  knowing  the 
velocity  of  the  ball,  to  calculate  the  attraction  of  the 
earth.  The  moon  is  a  satellite,  moving  like  such  a  ball, 
and  an  observer  on  Mars  would  be  able,  by  measuring 
the  orbit  of  the  moon,  to  determine  the  attraction  of  the 
earth  as  well  as  we  determine  it  by  actually  observing 
the  motion  of  falling  bodies  around  us. 

Thus  it  is  that  when  a  planet  like  Mars  or  Jupiter  has 
satellites  revolving  around  it,  astronomers  on  the  earth 
can  observe  the  attraction  of  the  planet  on  its  satellites 
and  thus  determine  its  mass.  The  rule  for  doing  this  is 
very  simple.  The  cube  of  the  distance  between  the  planet 
and  satellite  is  divided  by  the  square  of  the  time  of  revo- 
lution. The  quotient  is  a  number  which  is  propor- 
tional to  the  mass  of  the  planet.  The  rule  applies 
to  the  motion  of  the  moon  round  the  earth  and  of 
the  planets  round  the  sun.  If  we  divide  the  cube 
of  the  earth's  distance  from  the  sun,  say  ninety-three 
millions  of  miles,  by  the  square  of  three  hundred  and 
sixty-five  and  a  quarter,  the  days  in  a  year,  we  shall  got 
a  certain  quotient.  Let  us  call  this  number  the  sun- 
quotient.  Then,  if  we  divide  the  cube  of  the  moon's  dis- 
tance from  the  earth  by  the  square  of  its  time  of  revolu- 
tion, we  shall  get  another  quotient,  which  we  may  call 
the  earth-quotient.  The  sun-quotient  will  come  out 
about  three  hundred  and  thirty  thousand  times  as  large 
as  the  earth-quotient.  Hence  it  is  concluded  that  the 
mass  of  the  sun  is  three  hundred  and  thirty  thousand 


HOW  THE  PLANETS  ARE  WEIGHED  251 

times  that  of  the  earth;  that  it  would  take  this  number 
of  earths  to  make  a  body  as  heavy  as  the  sun. 

I  give  this  calculation  to  illustrate  the  principle;  it 
must  not  be  supposed  that  the  astronomer  proceeds  ex- 
actly in  this  way  and  has  only  this  simple  calculation  to 
make.  In  the  case  of  the  moon  and  earth,  the  motion  and 
distance  of  the  former  vary  in  consequence  of  the  attrac- 
tion of  the  sun,  so  that  their  actual  distance  apart  is  a 
changing  quantity.  So  what  the  astronomer  actually 
does  is  to  find  the  attraction  of  the  earth  by  observing 
the  length  of  a  pendulum  which  beats  seconds  in  various 
latitudes.  Then  by  very  delicate  mathematical  processes 
he  can  find  with  great  exactness  what  would  be  the  time 
of  revolution  of  a  small  satellite  at  any  given  distance 
from  the  earth,  and  thus  can  get  the  earth-quotient. 

But,  as  I  have  already  pointed  out,  we  must,  in  the 
case  of  the  planets,  find  the  quotient  in  question  by  means 
of  the  satellites ;  and  it  happens,  fortunately,  that  the 
motions  of  these  bodies  are  much  less  changed  by  the  at- 
traction of  the  sun  than  is  the  motion  of  the  moon.  Thus, 
when  we  make  the  computation  for  the  outer  satellite  of 
Mars,  we  find  the  quotient  to  be  ^o^sTsTo  that  °f  *ne 
sun-quotient.  Hence  we  conclude  that  the  mass  of  Mars 
is  3,o93,5oo  that  of  the  sun.  By  the  corresponding  quo- 
tient, the  mass  of  Jupiter  is  found  to  be  about  1<047 
that  of  the  sun ;  Saturn,  3,5*00  »  Uranus,  22,700?  Neptune, 


i 

1  9,500 


I  have  set  forth  only  the  great  principle  on  which  the 
astronomer  has  proceeded  for  the  purpose  in  question. 
The  law  of  gravitation  is  at  the  bottom  of  all  his  work. 


252    PLANETS  AND  THEIR  SATELLITES 

The  effects  of  this  law  require  mathematical  processes 
which  it  has  taken  two  hundred  years  to  bring  to  their 
present  state,  and  which  are  still  far  from  perfect.  The 
measurement  of  the  distance  of  a  satellite  is  not  a  job 
to  be  done  in  an  evening;  it  requires  patient  labor  ex- 
tending through  months  and  years,  and  then  is  not  as 
exact  as  the  astronomer  would  wish.  He  does  the  best 
he  can  and  must  be  satisfied  with  the  result  until  he  can 
devise  an  improvement  on  his  work,  which  he  is  always 
trying  to  do  with  varying  success. 


PART   V 
COMETS    AND    METEORIC   BODIES 


r 

COMETS 

COMETS  differ  from  the  heavenly  bodies  which  we  have 
hitherto  studied  in  their  peculiar  aspects,  their  eccentric 
orbits,  and  the  rarity  of  their  appearance.  Some  mystery 
still  surrounds  the  question  of  their  constitution,  but  this 
does  not  detract  from  the  interest  of  the  phenomena 
which  they  present.  When  one  of  these  objects  is  care- 
fully examined  we  find  it  to  embody  three  features  which, 
however,  are  not  separate  and  distinct,  but  merge  into 
each  other. 

First  we  have  what,  to  the  naked  eye,  appears  to  be 
a  star  of  greater  or  less  brilliancy.  This  is  called  the 
nucleus  of  the  comet. 

Surrounding  the  nucleus  is  a  cloudy  nebulous  mass, 
like  a  little  bunch  of  fog,  shading  off  very  gradually  to- 
ward the  edge,  so  that  we  cannot  exactly  define  its  bound- 
ary. This  is  called  the  coma  (Latin  for  hair).  Nucleus 
and  coma  together  are  called  the  head  of  the  comet, 
which  looks  like  a  star  shining  through  a  patch  of  mist 
or  fog. 

Stretching  away  from  the  comet  is  the  tail,  which  may 
be  of  almost  any  length.  In  small  comets  the  tail  may  be 
ever  so  short,  while  in  the  greatest  it  stretches  over  a  long 
arc  of  the  heavens.  It  is  narrow  and  bright  near  the  head 
of  the  comet  and  grows  wider  and  more  diffuse  as  it 


256      COMETS  AND  METEORIC  BODIES 

recedes  from  the  head.  It  is  therefore  always  more  or 
less  fan-shaped.  Toward  the  end  it  fades  away  so  gradu- 
ally that  it  is  impossible  to  say  how  far  the  eye  can 
trace  it. 

Comets  differ  enormously  in  brightness,  and,  notwith- 
standing the  splendid  aspect  which  the  brighter  ones  as- 
sume, the  great  majority  of  these  objects  are  quite  invis- 
ible to  the  naked  eye.  Such  are  called  telescopic  comets. 
There  is,  however,  no  broad  distinction  to  be  drawn  be- 
tween a  telescopic  comet  and  a  bright  one,  there  being 
a  regular  range  of  brightness  from  the  faintest  of  these 
objects  to  the  most  brilliant.  Sometimes  a  telescopic 
comet  has  no  visible  tail ;  this,  however,  is  the  case  only 
when  the  object  is  extremely  faint.  Sometimes,  also,  the 
nucleus  is  almost  wholly  wanting.  In  such  a  case  all 
that  can  be  seen  is  a  small  hairy  mass,  like  a  very  thin 
cloud,  which  may  be  a  little  brighter  in  the  centre. 

From  the  historical  records  it  would  appear  that  from 
twenty  to  thirty  comets  visible  to  the  naked  eye  gener- 
ally appear  in  the  course  of  a  century.  But  when  the 
telescope  was  employed  in  sweeping  the  heavens  it  was 
found  that  these  objects  were  more  numerous  than  had 
been  supposed.  Quite  a  number  are  now  found  every 
year  by  diligent  observers.  Doubtless  the  number  de- 
pends very  largely  on  accident,  as  well  as  on  the  skill 
applied  in  the  search.  Sometimes  the  same  comet  will  be 
found  independently  by  several  observers.  The  credit  is 
then  given  to  the  one  who  first  accurately  fixes  the  posi- 
tion of  the  comet  at  a  given  time,  and  telegraphs  the  fact 
to  an  observatory. 


ORBITS    OF    COMETS 


257 


Orbits  of  Comets 

Soon  after  the  invention  of  the  telescope  it  was  found 
that  comets  resembled  the  planets  in  moving  in  orbits 
around  the  sun.  Sir  Isaac  Newton  showed  that  their 
motions  were  ruled  by  the  sun's  gravitation  in  the  same 
way  as  the  motions  of  the  planets.  The  great  difference 
was  that,  instead  of  the  orbits  being  nearly  circular,  like 
those  of  the  planets,  they  were  so  elongated  that,  in  most 
cases,  it  could  not  be  determined  where  the  aphelion,  or 
farther  end,  was.  As  many  of  our  readers  may  desire 
an  exact  statement  of  the  nature  of  cometary  orbits,  and 
the  laws  governing  them,  we  shall  enter  into  some 
explanations  of  the  subject. 

It  was  shown  by  Newton 
that  a  body  moving  under 
the  influence  of  the  sun's  at- 
traction would  always  de- 
scribe a  conic  section.  This 
curve  is  of  three  kinds,  an 
ellipse,  a  parabola,  and  a 
hyperbola.  The  first,  as  we 
all  know,  is  a  closed  curve 
returning  into  itself.  But 
the  parabola  and  the  hyper- 
bola are  not  such ;  each  of  them  extends  out  without  end 
in  two  branches.  In  the  case  of  the  parabola  these  two 
branches  approach  more  nearly  to  having  the  same  direc- 
tion as  we  get  out  farther,  but  in  the  case  of  the  hyper- 
bola they  always  diverge  from  each  other. 


FIG.  45. — Parabolic   Orbit  of  a 
Comet. 


258      COMETS  AND  METEORIC  BODIES 

Having  these  curves  in  mind,  let  us  imagine  the  earth 
to  leave  us  hanging  in  space  at  some  point  of  its  orbit, 
our  planet  pursuing  its  course  without  us,  until,  at  the 
end  of  a  year,  it  returns  to  pick  us  up  again.  During 
the  interval  of  its  absence  we,  hanging  in  mid-space, 
amuse  ourselves  by  firing  off  balls  to  perform  their  revo- 
lutions around  the  sun  like  little  planets.  The  result  will 
be  that  all  the  balls  we  send  off  with  a  velocity  less  than 
that  of  the  earth,  that  is  to  say,  less  than  eighteen  and 
six  "tenths  miles  per  second,  will  move  around  the  sun  in 
closed  orbits,  smaller  than  the  orbit  of  the  earth,  no  mat- 
ter what  direction  we  send  them  in.  A  very  simple  and 
curious  law  is  that  these  orbits  will  always  have  the  same 
period  if  the  velocity  is  the  same.  All  the  balls  sent  with 
the  velocity  of  the  earth  will  be  one  year  in  making  their 
revolution  and  will,  therefore,  come  together,  at  the  point 
from  which  they  started,  at  the  same  moment.  If  the 
velocity  exceeds  eighteen  and  six  tenths  miles  a  second,  the 
orbit  will  be  larger  than  that  of  the  earth  and  the  period 
of  revolution  will  be  longer  the  greater  the  velocity. 
With  a  speed  exceeding  about  twenty-six  miles  a  second, 
the  attraction  of  the  sun  could  never  hold  in  the  ball, 
which  would  fly  away  for  good  in  one  of  the  branches  of 
a  hyperbola.  This  would  happen  no  matter  in  what 
direction  we  threw  the  object.  There  is,  therefore,  at 
every  distance  from  the  sun,  a  certain  limiting  velocity 
which,  if  a  comet  exceeds,  it  will  fly  off  from  the  sun 
never  to  return ;  while,  if  it  falls  short,  it  will  be  sure  to 
get  back  at  some  time. 

The  nearer  we  are  to  the  sun,  the  greater  is  this  limit- 


ORBITS    OF    COMETS  259 

ing  velocity.  It  varies  inversely  as  the  square  root  of 
the  distance  from  the  sun,  hence,  four  times  away  from 
the  sun,  it  is  only  half  as  great.  The  rule  for  finding 
the  limiting  velocity  at  any  point  in  space  is  very  simple. 
It  is  to  take  the  speed  of  a  planet  passing  through  that 
point  in  a  circular  orbit,  and  multiply  it  by  the  square 
root  of  2.  This  is  1.414.  .  .  . 

It  follows  that  if  the  astronomer,  by  means  of  his  ob- 
servations, can  find  the  velocity  with  which  a  comet  is 
passing  a  known  point  of  its  orbit,  he  can  determine  the 
distance  to  which  it  will  fly  from  the  sun  and  the  period 
of  its  return.  By  a  careful  comparison  of  observation 
made  during  the  whole  period  of  visibility  of  the  comet 
he  can  generally  reach  a  definite  conclusion  on  the 
subject. 

It  is  a  curious  fact  that  no  comet  nas  yet  been  seen  of 
which  the  speed  certainly  exceeds  the  limit  which  we  have 
described.  It  is  true  that,  in  many  cases,  a  slight  excess 
has  been  calculated  from  the  observations,  but  this  excess 
was  no  greater  than  might  result  from  the  necessary 
errors  of  observations  on  bodies  of  this  kind.  Commonly 
the  speed  is  so  near  the  limit  that  it  is  impossible  to  say 
whether  it  exceeds  it  or  not.  It  is  then  certain  that  the 
comet  will  fly  out  to  an  immense  distance,  not  returning 
for  hundreds,  thousands,  or  tens  of  thousands  of  years. 
There  are  also  cases  in  which  the  speed  of  the  comet  is 
found  to  be  less  than  the  limit  by  a  considerable  amount. 
Such  comets  complete  their  revolutions  in  shorter  period? 
and  are  called  periodic  comets. 

So  far  as  we  know,  the  history  of  the  motion  of  the 


260      COMETS  AND  METEORIC  BODIES 

large  majority  of  the  comets  is  this,  They  appear  to 
us  as  if  falling  toward  the  sun  from  some  great  distance, 
we  know  not  what.  If  a  comet  fell  exactly  toward  the 
sun,  it  would  fall  into  it,  but  this  is  a  case  which  has  not 
been  known  to  occur  and  which,  for  reasons  to  be  ex- 
plained later,  cannot  be  expected  ever  to  occur.  As  it 
approaches  the  sun,  it  acquires  greater  and  greater 
velocity,  speeds  around  the  central  body  in  a  great  curve, 
and,  by  the  centrifugal  force  thus  generated,  flies  off 
again,  returning  to  the  abyss  of  space  nearly  in  the 
direction  from  which  it  came. 

Owing  to  the  faintness  of  these  objects  they  are  visible, 
even  in  powerful  telescopes,  only  in  that  part  of  their 
orbit  which  is  comparatively  near  the  sun.  This  is  what 
makes  it  so  difficult  in  many  cases  to  determine  the  exact 
period  of  a  comet  which  has  only  been  seen  once. 

H alley's  Comet 

The  first  of  these  objects  which  was  found  to  return 
in  a  regular  period  is  celebrated  in  the  history  of  astron- 
omy under  the  name  of  Halley's  comet.  It  appeared  in 
August,  1682,  and  was  observed  for  about  a  month,  when 
it  disappeared  from  view.  Halley  was  able,  from  the  ob- 
servations made  upon  it,  to  compute  the  position  of  the 
orbit.  He  found  that  the  latter  was  in  the  same  position 
as  that  of  a  bright  comet  observed  by  Kepler  in  1607. 

It  did  not  seem  at  all  likely  that  two  comets  should 
move  precisely  in  the  same  orbit.  Halley  therefore 
judged  that  the  real  orbit  was  an  ellipse,  and  that  the 
comet  had  a  period  of  about  seventy-five  years.  If  this 


HALLEY'S    COMET  261 

were  the  case,  it  should  have  been  visible  at  intervals  of 
about  seventy-five  years  in  the  past. 

So  he  subtracted  this  period  from  the  several  dates  in 
order  to  determine  whether  any  comets  were  recorded. 
Subtracting  seventy-five  from  1607  we  have  1532.  He 
found  that  a  comet  had  actually  appeared  in  1531,  which 
he  had  reason  to  believe  was  moving  in  the  same  orbit. 
Again  subtracting  seventy-five  from  this  year  we  have 
the  year  1456.  A  comet  really  did  appear  in  1456,  which 
spread  such  horror  throughout  Christendom  that  Pope 
Calixtus  III  ordered  prayers  to  be  offered  for  protection 
against  the  comet  as  well  as  against  the  Turks,  who  were 
at  war  against  Europe.  It  is  probable  that  the  n^th 
of  "the  Pope's  Bull  against  the  comet"  refers  to  this 
circumstance. 

Other  possible  appearances  of  the  comet  were  found  in 
past  history,  but  Halley  was  not  able  to  identify  the 
comet  with  exactness,  owing  to  the  absence  of  any  pre- 
cise description  of  the  body.  But  the  four  well-observed 
dates,  1456,  1531,  1607,  and  1682,  afforded  ample 
ground  for  predicting  that  the  comet  would  again  return 
to  the  sun  about  1758.  Clairaut,  one  of  the  most  eminent 
mathematicians  then  in  France,  was  able  to  calculate  wb«\t 
effect  would  be  produced  by  the  action  of  Jupiter  «u.d 
Saturn  on  the  period  of  the  comet.  He  found  that  this 
action  would  so  delay  its  return  that  it  would  not  reach 
perihelion  until  the  spring  of  1759.  It  appeared  accord- 
ing to  the  prediction,  and  actually  passed  perihelion  on 
March  twelfth  of  that  year. 

The    next   predicted   return   was   in    1835.      Several 


262      COMETS  AND  METEORIC  BODIES 

mathematicians  now  made  computations  of  the  effect  of 
the  planets  in  changing  its  period.  So  exact  was  their 
work  that  two  of  them  hit  the  time  within  five  days  :  Pro- 
fessor Rosenberger  assigned  November  eleventh  as  the 
date  of  return,  and  Pontecoulant  predicted  it  for  Novem- 
ber thirteenth.  It  actually  passed  perihelion  on  November 
sixteenth.  After  being  observed  for  several  months  it 
disappeared  from  view  and  has  not  since  been  seen.  But 
so  exact  is  astronomical  science  that  an  astronomer  could, 
at  any  time  during  the  intervening  interval,  have  pointed 
his  telescope  exactly  at  the  object,  after  making  the 
necessary  calculations  to  determine  its  position. 

Its  next  return  is  now  approaching,  but  the  exact  date 
has  not  yet  been  computed.  It  will  probably  be  some 
time  between  1910  and  1912. 


Comets  which  have  Disappeared 

The  most  striking  discovery  of  a  comet  after  Halley 
announced  the  one  which  bears  his  name,  was  made 
by  the  French  astronomer  Lexell,  in  June,  1770.  The 
object  soon  became  visible  to  the  naked  eye.  On  laying 
down  the  orbit  in  which  it  moved,  it  was  found,  to  the 
surprise  of  astronomers,  that  the  orbit  was  an  ellipse, 
with  a  period  of  only  about  six  years.  Its  return  was, 
therefore,  confidently  predicted,  but  it  never  reappeared. 
The  cause  was,  however,  speedily  discovered.  When  it 
returned  at  the  end  of  six  years,  it  was  on  the  opposite 
side  of  the  sun,  and  therefore  could  not  be  seen.  Passing 
out  to  complete  its  revolution,  it  was  found  by  calculation 
that  it  must  have  gone  into  the  immediate  neighbourhood 


COMETS  WHICH  HAVE  DISAPPEARED  263 

of  the  planet  Jupiter,  which,  by  its  powerful  attraction, 
started  the  comet  off  into  some  new  orbit,  so  that  it  never 
again  came  within  reach  of  the  telescope.  This,  also, 
explained  why  the  comet  had  not  been  seen  before.  Three 
years  before  Lexell  found  it,  it  had  come  from  the  neigh- 
bourhood of  the  planet  Jupiter,  which  had  thrown  it  into 
an  orbit  different  from  its  former  one.  Thus  the  giant 
planet  of  our  system  had,  so  to  speak,  given  the  comet  a 
pull  about  1767  so  that  it  should  pass  into  the  immediate 
neighbourhood  of  the  sun,  and  having  allowed  it  to  make 
two  revolutions  around  the  sun,  again  encountered  it  in 
1779,  and  gave  it  a  new  swing  off,  no  one  knows  where. 
Since  that  time  twenty  or  thirty  comets,  found  to  be 
periodic,  have  been  observed,  most,  but  not  all  of  them, 
at  two  or  more  returns. 

The  most  remarkable  fact  brought  out  by  the  study 
of  these  objects  has  been  that  they  do  not  appear  to  be 
of  seemingly  infinite  duration,  like  the  planets,  but  are, 
as  a  general  rule,  subject  to  dissolution  and  decay,  like 
living  beings.  The  most  curious  case  of  a  comet  being 
completely  disintegrated  is  that  of  Biela's  comet.  This 
jvas  first  observed  in  1772,  but  was  not  known  to  be  peri- 
odic. It  was  again  seen  in  1805,  and  again  the  astrono- 
mer did  not  notice  the  identity  of  the  orbit  in  which  it  was 
moving  with  that  of  the  comet  of  1772.  In  1826  it  was 
discovered  a  third  time,  and  now,  on  computing  the  orbit 
by  the  improved  methods  which  had  been  invented,  its 
identity  with  the  former  comets  was  brought  out.  The 
time  of  revolution  was  fixed  at  six  and  two  thirds  years. 
It  should,  therefore,  appear  in  1832  and  1839.  But  on 


264      COMETS  AND  METEORIC  BODIES 

these  returns  the  earth  was  not  in  a  position  to  admit  of 
its  being  seen.  Toward  the  end  of  1845  it  again  ap- 
peared and  was  observed  in  November  and  December. 
In  January,  1846,  as  it  came  nearer  to  the  earth  and  sun, 
it  was  found  to  have  separated  into  two  distinct  bodies. 
At  first  the  smaller  of  these  was  quite  faint,  but  it  seemed 
to  increase  in  brightness  until  it  became  equal  to  the  other. 

The  next  return  was  in  1852.  The  two  bodies  were 
then  found  to  be  far  more  widely  separated  than  before. 
In  1846  their  distance  apart  was  about  two  hundred 
thousand  miles ;  in  1852  more  than  a  million  miles.  The 
last  observations  were  made  in  September,  1852.  Al- 
though since  that  time  the  comet  should  have  completed 
seven  revolutions,  it  has  never  again  been  seen.  From  the 
former  returns  it  was  possible  to  compute  the  position 
where  it  should  appear  with  a  good  deal  of  precision,  and 
from  its  non-appearance  we  conclude  that  it  has  been 
completely  disintegrated.  We  shall,  in  the  next  chapter, 
learn  a  little  more  about  the  matter  which  composed  it. 

Two  or  three  comets  have  disappeared  in  the  same  way. 
They  were  observed  for  one  or  more  revolutions,  growing 
fainter  and  more  attenuated  on  each  occasion,  and  finally 
became  completely  invisible. 

EncJce's  Comet 

Of  the  periodic  comets  the  one  that  is  most  frequently 
and  regularly  observed  bears  the  name  of  Encke,  the 
German  astronomer  who  first  accurately  determined  its 
motion.  Its  first  discovery  was  made  in  1786,  but,  as 
was  often  the  case  then,  its  orbit  could  not  at  first  be 


ENCKE'S    COMET  265 

determined.  It  was  again  seen  in  1795  by  Miss  Caroline 
Herschel.  It  was  found  again  in  1805  and  1818.  Not 
until  the  latter  date  was  the  accurate  orbit  determined, 
and  then  the  periodic  character  of  the  comet  and  its  iden- 
tity with  the  comet  observed  in  previous  years  was 
established. 

Encke  now  found  the  period  to  be  about  three  years 
and  one  hundred  and  ten  days,  varying  a  little  according 
to  the  attraction  of  the  planets,  especially  of  Jupiter. 
In  recent  times  it  has  been  observed  somewhere  at  almost 
every  return.  Its  last  return  was  in  September,  1901. 

What  has  given  this  comet  its  celebrity  is  the  theory  of 
Encke  that  its  orbit  was  continually  becoming  smaller, 
probably  through  its  motion  being  resisted  by  some 
medium  surrounding  the  sun.  A  number  of  able  mathe- 
maticians have  investigated  this  subject  on  the  various 
returns  of  the  comet.  Sometimes  there  appears  to  be 
evidence  of  a  retardation,  like  that  found  by  Encke,  and 
sometimes  not.  The  question  is,  therefore,  still  in  an  un- 
settled condition.  The  computations  are  so  intricate  and 
difficult,  and,  indeed,  the  whole  problem  of  the  motion 
of  a  comet  under  the  influence  of  the  planets  is  so  compli- 
cated, that  it  is  almost  impossible  to  secure  a  solution 
which  can  be  guaranteed  as  absolutely  correct. 

Capture  of  Comets  by  Jupiter 

A  remarkable  case,  in  which  a  new  comet  was  made 
a  member  of  the  solar  system,  occurred  in  the  years  1886- 
1889.  In  the  latter  year  a  comet  was  observed  by  Brooks 
of  Geneva,  New  York,  which  proved  to  be  revolving  in 


266      COMETS  AND  METEORIC  BODIES 

an  orbit  with  a  period  of  only  seven  years.  As  it  was 
quite  bright,  the  question  arose  why  it  had  never  been 
observed  before.  This  question  was  soon  answered  by 
the  discovery  that  in  the  year  1886  the  comet  had  passed 
close  to  Jupiter.  The  attraction  of  the  planet  had  so 
changed  its  course  as  to  throw  the  comet  into  the  orbit 
which  it  now  describes.  Several  other  periodic  comets 
pass  so  near  to  Jupiter  that  there  is  little  doubt  that  they 
were  brought  into  the  system  in  this  way. 

The  question  therefore  arises  whether  this  may  not  be 
true  of  all  periodic  comets.  This  question  must  be  an- 
swered in  the  negative,  because  Halley's  comet  does  not 
pass  near  any  planet.  The  same  is  true  of  Encke's 
comet,  which  does  not  come  near  enough  to  the  orbit  of 
Jupiter  to  have  been  drawn  into  its  present  orbit.  With- 
out the  action  of  that  planet,  so  far  as  we  know,  these 
comets  always  have  been  members  of  the  system. 

Whence  Come  Comets? 

It  was  supposed,  until  a  recent  time,  that  comets  might 
come  into  the  solar  system  from  the  vast  spaces  between 
the  stars.  This  view,  however,  seems  to  be  set  aside 
by  the  fact  that  no  comet  has  been  proved  to  move  with 
a  much  higher  speed  than  it  would  get  by  falling  to  the 
sun  from  a  distance,  which,  though  far  outside  the  solar 
system,  is  much  less  than  the  distance  of  the  stars.  .  We 
shall  see  hereafter  that  the  sun  itself  is  in  motion  through 
space.  Even  if  we  grant  that  comets  come  from  space 
far  outside  the  solar  system,  the  fact  that  we  have  just 
cited  still  shows  that  they  partook  of  the  motion  of  the 


WHENCE    COME    COMETS?  267 

sun  and  solar  system  through  space  while  they  were  still 
outside  that  system. 

The  view  which  now  seems  established  by  a  study  of 
the  whole  subject  is  that  these  objects  have  their  regular 
orbits,  differing  from  those  of  the  planets  in  their  great 
eccentricities.  Their  periods  of  revolution  are  generally 
thousands,  and  sometimes  tens  of  thousands,  and  even 
hundreds  of  thousands  of  years.  During  this  long  inter- 
val they  fly  out  to  an  enormous  distance  beyond  the  con- 
fines of  the  system.  If,  as  they  return  to  the  sun,  they 
chance  to  pass  very  near  a  planet,  two  things  may  hap- 
pen :  Either  the  comet  may  be  given  an  additional  swing 
that  will  accelerate  its  speed,  throw  it  out  to  a  greater 
distance  than  it  ever  had  before  or  possibly  to  a  distance 
from  which  it  can  never  return,  or  the  speed. may  be  re- 
tarded and  the  comet  made  to  move  in  a  smaller  orbit. 
Thus  it  is  that  we  have  comets  of  so  many  different 
periods.  If  comets  come  from  the  regions  of  the  fixed 
stars,  there  is  no  reason  why  the  motion  of  one  might 
not  be  directly  toward  the  sun,  so  that  it  would  fall  into 
our  central  luminary.  But  such  an  occurrence  is  hardly 
possible  when  the  comet  belongs  to  our  system,  because 
one  of  these  bodies  nearing  an  orbit  passing  through  the 
sun  would  have  fallen  into  the  sun  on  its  first  round,  long 
ages  ago,  and  never  could  have  a  chance  to  fall  in  again. 

Brilliant  Comets  of  Our  Time 

The  very  bright  comets  which  appear  from  time  to 
time  are  of  the  greatest  interest  to  every  beholder.  It  is 
purely  a  matter  of  chance,  so  far  as  our  knowledge  ex- 


268      COMETS  AND  METEORIC  BODIES 


FiG.  46. — Donates  Comet,  as  drawn  by  G.  P.  Bond. 


BRILLIANT  COMETS  OF  OUR  TIME    269 

tends,  when  one  shall  appear.  Of  what  are  called  great 
comets,  there  were  five  or  six  during  the  nineteenth  cen- 
tury. The  most  remarkable  and  brilliant  of  all  appeared 
in  1858,  and  bears  the  name  of  Donati,  its  discoverer,  an 
astronomer  of  Florence,  Italy.  Its  history  will  show  the 
changes  through  which  such  a  body  goes.  It  was  first 
seen  on  June  second,  but  was  then  only  a  faint  nebulosity, 
visible  in  the  telescope  like  a  minute  white  cloud  in  the 
heavens.  No  tail  was  then  visible,  nor  was  there  the 
slightest  indication  of  what  the  little  cloud  would  grow 
into  until  the  middle  of  August.  Then  a  small  tail 
gradually  began  to  form.  Early  in  September  the  ob- 
ject  became  visible  to  the  naked  eye.  From  that  time  it 
increased  at  an  extraordinary  rate,  growing  larger  and 
more  conspicuous  night  after  night.  Its  motions  wrere 
such  that  it  seemed  to  move  but  little  for  the  period  of  a 
whole  month,  floating  in  the  western  sky  night  after 
night.  It  attained  its  greatest  brilliancy  about  October 
tenth.  Careful  drawings  of  it  were  made  from  time  to 
time  by  George  P.  Bond,  of  the  Harvard  Observatory. 
We  give  two  of  these,  one  a  naked  eye  view,  the  other  a 
telescopic  one  showing  what  the  head  of  the  comet  looked 
like.  After  October  tenth  it  rapidly  faded  away.  It  soon 
travelled  toward  the  south,  and  passed  below  our  horizon, 
but  was  followed  by  observers  in  the  southern  hemisphere 
until  March,  1859. 

Before  the  comet  had  passed  out  of  sight,  computers 
began  to  calculate  its  orbit.  It  was  soon  found  not  to 
move  in  an  exact  parabola,  but  in  a  very  elongated  ellipse. 
The  period  was  not  far  from  nineteen  hundred  years,  but 


270      COMETS  AND  METEORIC  BODIES 


FIG.  47. — Head  of  Donates  Comet,  drawn  by  G.  P.  Bond. 


BRILLIANT  COMETS  OF  OUR  TIME    271 

may  have  been  a  hundred  years  more  or  less  than  this.  It 
must  therefore  have  been  visible  at  its  preceding  return 
sometime  in  the  first  century  before  Christ,  but  there  is 
no  record  by  which  it  could  be  identified.  It  may  be 
expected  again  in  the  thirty-eighth  or  thirty-ninth 
century. 

A  very  remarkable  case  of  several  comets  moving  in 
very  nearly  the  same  orbit  is  afforded  by  the  comets  of 
1843,  1880,  and  1882.  The  first  of  these  was  one  of  the 
most  memorable  comets  on  record,  as  it  passed  so  near 
the  sun  as  almost  to  graze  the  surface.  In  fact,  it  must 
have  passed  quite  through  the  outer  portions  of  the 
solar  corona.  It  came  into  view  with  remarkable  sudden- 
ness in  the  neighbourhood  of  the  sun,  about  the  end  of 
February.  It  was  visible  in  full  daylight.  By  a  singular 
coincidence  it  appeared  shortly  after  the  well-known  pre- 
diction of  Miller  that  the  end  of  the  world  was  to  come 
in  the  year  1843.  Those  who  had  been  alarmed  by  this 
prediction  saw  in  the  comet  an  omen  of  the  approaching 
catastrophe. 

The  comet  disappeared  from  view  in  April,  so  that  the 
time  of  observation  was  rather  short.  The  period  of 
revolution  now  became  a  subject  of  interest.  It  was 
found,  however,  that  its  orbit  did  not  differ  sensibly  from 
the  parabola.  But  the  time  of  observation  was  so  brief 
that  any  estimate  of  the  period  would  be  somewhat  un- 
certain. All  that  could  be  said  was  that  the  comet  would 
not  return  for  several  centuries. 

Great,  therefore,  was  the  surprise  when,  thirty-seven 
years  later,  a  comet  was  seen  by  observers  in  the  southern 


COMETS  AND  METEORIC  BODIES 


FIG.  48.— Great  Comet  of  1859,  drawn  by  G.  P.  Bond. 


BRILLIANT  COMETS  OF  OUR  TIME    273 

hemisphere  and  found  to  be  moving  in  almost  the  same 
orbit.  The  first  sign  which  it  gave  of  its  approach  was 
its  long  tail  rising  above  the  horizon.  This  was  seen  in 
the  Argentine  Republic,  at  the  Cape  of  Good  Hope,  and 
in  Australia.  Not  until  the  fourth  of  February  did  the 
head  become  visible.  It  swept  around  the  sun,  again 
passed  to  the  south,  and  disappeared  without  observers 
in  the  northern  hemisphere  seeing  it. 

The  question  now  arose  whether  this  could  possibly  be 
the  same  comet  that  had  appeared  in  1843.  Previously 
it  had  been  supposed  that  when  two  such  bodies  moved  in 
the  same  orbit  with  a  long  interval  between  they  must  be 
the  same.  In  the  present  case,  however,  the  hypothesis  of 
identity  seemed  to  be  incompatible  with  the  observations. 
The  question  was  set  at  rest  by  the  appearance  in  1882 
of  a  third  comet  moving  in  about  the  same  orbit.  This 
certainly  could  not  be  a  return  of  the  comet  which  had 
appeared  a  little  more  than  two  years  before.  The  re- 
markable spectacle  was  therefore  offered  of  three  bright 
comets  all  moving  in  the  same  orbit  at  varying  intervals 
of  time.  Possibly  there  were  more  even  than  these  three, 
for,  in  1680,  a  comet  had  passed  very  near  the  sun.  Its 
orbit,  however,  was  spmewhat  different  from  those  of  the 
three  comets  already  mentioned. 

The  most  probable  explanation  of  the  .case  seems  to  be 
that  these  comets  were  parts  of  some  nebulous  mass  which 
gradually  broke  up,  its  different  members  pursuing  their 
courses  independently.  The  result  would  be  that,  for 
many  ages,  the  objects  would  all  continue  in  nearly  the 
same  orbit 


274      COMETS  AND  METEORIC  BODIES 

Besides  these,  brilliant  comets  appeared  in  1859,  1860, 
and  1881.  How  long  we  may  have  to  wait  for  another 
no  one  can  say.  It  is  probable  that  Halley's  comet,  when 
it  appears  eight  or  ten  years  hence,  will  at  least  be  visible 
to  the  naked  eye,  but  no  one  can  predict  even  its  apparent 
brightness.  At  its  return  in  1835  it  was  so  small  an 
affair  that  it  was  difficult  to  explain  the  excitement  it 
caused  in  1406  and  later,  except  by  supposing  a  great 
diminution  in  the  dimensions,  at  least  of  its  tail. 

Nature  of  Comets 

The  question  of  the  exact  nature  of  a  comet  is  still  in 
doubt.  In  the  case  of  large  and  bright  comets,  it  is  possi- 
ble that  the  nucleus  may  be  a  solid  body,  though  probably 
much  smaller  than  it  looks.  Some  light  on  the  question 
is  thrown  by  an  observation,  which  is  unique,  made  at  the 
Cape  of  Good  Hope  when  the  great  comet  of  1882 
made  a  transit  across  the  sun's  disk,  as  Mercury  and 
Venus  are  sometimes  known  to  do.  Unfortunately,  as- 
tronomers generally  were  not  prepared  for  such  a  phe- 
nomenon, as  the  comet  had  been  visible  only  in  the 
southern  hemisphere,  and  the  transit  occurred  only  a 
week  or  two  after  its  first  discovery.  Hence  it  happened 
that  the  Cape  Observatory  was  the  only  one  at  which  an 
observation  of  the  greatest  interest  in  astronomy  could 
be  made ;  and  here  the  circumstances  were  extremely  un- 
favourable. The  sun  was  about  to  set  behind  Table  Moun- 
tain as  the  comet  approached  it.  By  careful  watching, 
two  of  the  astronomers,  Messrs.  Finlay  and  Elkin,  were 
enabled  to  keep  sight  of  the  comet  until  it  actually  disap- 


NATURE    OF    COMETS  275 

pcarcd  at  the  limb  of  the  sun.  This  happened  fifteen 
minutes  before  the  sun  disappeared  from  view.  During 
this  time,  if  the  nucleus  were  a  solid  body,  it  ought  to 
have  been  seen  as  a  black  spot  projected  against  the  sun. 
Nothing  of  the  sort  could  be  made  out.  The  conclusion 
is  either  that  the  substance  of  the  comet  was  transparent 
to  the  sun's  rays,  or  that  the  solid  nucleus  was  too  small  to 
be  distinguished  under  the  circumstances.  Unfortunate- 
ly, owing  to  the  low  altitude  of  the  sun  and  the  bad  condi- 
tion of  the  air,  it  was  impossible  to  be  quite  sure  how 
small  the  nucleus  must  have  been  to  be  invisible.  It 
seemed  certain,  however,  that  the  solid  portion,  if  any 
such  the  comet  had,  was  much  smaller  than  the  apparent 
nucleus  as  seen  in  the  telescope. 

There  seems  also  to  be  some  reason  for  suspecting  that 
a  comet  is  nothing  but  a  collection  of  meteoric  matter, 
consisting  perhaps  of  separate  objects,  of  sizes  ranging 
anywhere  from  that  of  grains  of  sand  to  masses  as  large 
as  the  meteorites  which  sometimes  fall  from  the  sky.  The 
question  then  is  to  explain  how  the  parts  are  kept  to- 
gether through  so  many  revolutions  of  the  comet.  The 
^changes  of  shape  which  the  nucleus  often  undergoes  as  it 
is  passing  near  to  the  sun  seem  to  show  that  this  hypothe- 
sis may  be  near  the  truth. 

The  spectra  of  those  comets  whose  light  has  been 
analysed  by  the  spectroscope  are  remarkable  in  showing 
that  this  light  is  not  merely  reflected  sunlight.  The 
principal  feature  is  three  bright  bands,  which  bear  a 
striking  resemblance  to  those  given  by  the  compounds 
of  carbon  and  hydrogen.  Taking  this  fact  by  itself,  the 


276      COMETS  AND  METEORIC  BODIES 

conclusion  would  be  that  the  comet  is  a  glowing  gas, 
shining  as  incandescent  gases  do  in  our  chemical  labora- 
tories. That  such  should  be  the  case  and  the  whole  case 
seems  impossible  for  two  reasons.  The  comet  cannot  be 
hot  enough  to  glow;  and  its  light  fades  out  to  nothing 
as  it  recedes  from  the  sun.  The  most  likely  conclusion 
seems  to  be  that  the  action  of  the  sun's  rays  causes  a  glow 
through  some  process  which  has  not  yet  been  made  clear 
to  us. 

What  seems  certain  is  that  the  matter  of  which  a  bright 
comet  is  composed  is  volatile.  When  a  bright  comet  is 
carefully  scrutinised  with  a  telescope,  masses  of  vapour 
can  be  seen  from  time  to  time  slowly  rising  from  its  head 
in  the  direction  of  the  sun,  then  spreading  out  and  mov- 
ing away  from  the  sun  so  as  to  form  the  tail.  The  latter 
is  not  an  appendage  which  the  comet  carries  as  animals 
carry  their  tails,  but  is  like  a  stream  of  smoke  issuing 
from  a  chimney. 

It  frequently  happens  that  when  a  comet  is  first  dis- 
covered it  has  no  tail  at  all.  The  latter  begins  to  form 
when  the  sun  is  approached.  The  nearer  the  comet  ap- 
proaches the  sun,  and  the  greater  the  heat  to  which  it  is 
exposed,  the  more  rapidly  the  tail  develops.  All  this 
shows  that  the  matter  which  composes  a  great  comet  is 
in  part  volatile.  When  warmed  by  the  heat  of  the  sun  it 
begins  to  evaporate,  just  as  water  would  under  the  same 
circumstances.  The  steam  or  vapour  thus  arising  is  re- 
pelled by  the  sun,  so  as  to  form  a  stream  of  matter  issu- 
ing from  the  comet. 


II 

METEORIC    BODIES 

EVERY  reader  of  this  book  must  frequently  have  seen 
what  is  familiarly  called  a  "shooting  star" — an  object 
like  a  star,  which  darts  through  the  heavens  a  greater  or 
less  distance,  and  then  disappears.  These  objects  are,  in 
astronomy,  called  by  the  generic  name  of  meteors.  They 
are  of  every  degree  of  brightness,  but  the  brighter  they 
may  be,  the  more  rarely  they  appear.  One  who  is  out 
much  at  night  will  seldom  pass  a  year  without  seeing  such 
a  meteor  of  striking  brilliancy.  Once  or  twice  in  a  life- 
time he  will  see  one  that  illuminates  the  whole  sky  with 
its  light. 

On  almost  any  clear  night  in  the  year  a  watcher  may 
see  three  or  four  or  even  more  meteors  in  the  course  of  an 
hour.  Sometimes,  however,  they  are  vastly  more  numcr- 
-ous,  for  example,  between  the  tenth  and  fifteenth  of 
August,  more  and  brighter  ones  than  usual  will  be  seen. 
On  a  number  of  occasions  in  history  they  have  coursed  the 
heavens  in  such  numbers  as  to  fill  the  beholders  with  sur- 
prise and  terror.  There  were  remarkable  cases  of  this 
kind  in  1799  and  1833.  In  the  latter  year,  especially, 
the  negroes  of  the  South  were  so  terrified  that  the  recol- 
lection of  the  phenomenon  is  brought  down  by  tradition 
to  the  present  day. 


£78      COMETS  AND  METEORIC  BODIES 

Cause  of  Meteors 

The  cause  of  meteors  was  unknown  until  after  the  be- 
ginning of  the  nineteenth  century.  It  is  now,  however, 
well  made  out.  Besides  the  known  objects  of  the  solar 
system — planets,  satellites,  and  comets — there  are,  cours- 
ing through  space,  and  revolving  around  the  sun,  count- 
less millions  of  particles,  or  minute  collections  of  matter, 
too  small  to  be  seen  with  the  most  powerful  telescope. 
Quite  likely  the  greater  number  of  these  objects  are 
scarcely  larger  than  pebbles,  or  even  grains  of  sand.  The 
earth,  in  its  course  around  the  sun,  is  continually  encoun- 
tering them.  One  in  the  line  of  motion  of  the  earth 
may  have  a  velocity  amounting  to  many  miles  a  sec- 
ond ;  perhaps  ten,  twenty,  thirty,  or  even  forty.  Meet- 
ing the  atmosphere  with  this  immense  velocity  causes  the 
body  to  be  immediately  heated  to  so  high  a  temperature 
that  its  substance  dissolves  away  with  a  brilliant  effusion 
of  light  .no  matter  how  solid  it  may  be.  What  we  see 
is  the  course  of  a  particle  thus  burning  away  as  it  darts 
through  the  rare  regions  of  the  upper  atmosphere. 

Of  course,  a  meteor  will  appear  brighter  and  last 
longer  the  larger  and  solidcr  it  is.  Sometimes  it  is  so 
large  and  solid  that  it  comes  within  a  few  miles  of  the 
earth  before  being  finally  melted  and  dissolved  away. 
Then,  the  people  in  the  region  over  which  it  is  passing, 
see  a  remarkably  bright  meteor.  In  such  a  case  it  fre- 
quently happens  that  in  a  few  minutes  after  the  meteor 
has  passed  a  loud  explosion,  like  the  firing  of  a  cannon, 
is  heard  coming  from  the  region  through  which  it  passed. 


METEORIC    SHOWERS  279 

This  arises  from  the  concussion  of  the  air  compressed  by 
the  rapid  flight. 

In  rare  cases  the  mass  is  so  large  that  it  reaches  the 
earth  without  being  melted  or  evaporated.  Then  we  have 
the  fall  of  a  meteoric  stone,  as  it  is  called,  which  com- 
monly occurs  several  times  a  year  in  some  part  or  another 
of  the  world.  There  is  at  least  one  case  on  record  in 
which  a  man  was  killed  by  the  fall  of  such  a  body.  When 
these  stones  are  dug  up  they  are  found  to  be  composed 
mostly  of  iron.  Specimens  of  them  are  kept  in  our 
museums,  where  they  may  be  examined  by  anyone  who 
wishes  to  see  them.  Some  remarkable  ones  are  found  at 
the  Smithsonian  Institution,  Washington,  D.  C. 

How  these  objects  originated  we  cannot  say,  and  even 
a  guess  on  the  subject  would  be  hazardous.  When  found 
they  bear  marks  on  their  surface  of  having  been  melted ; 
this,  however,  is  a  natural  result  of  their  passage  through 
the  air,  by  which  the  surface  is  always  heated  far  above 
the  melting  point. 

Meteoric  Showers 

The  greatest  discovery  of  our  times  on  the  subject  of 
meteors  is  connected  with  the  meteoric  showers  already 
referred  to,  which  occur  at  certain  seasons  of  the  year. 
The  most  remarkable  of  these  occur  in  November,  and  the 
meteors  of  the  shower  are  called  Leonides,  because  their 
lines  of  apparent  motion  all  diverge  from  the  constella- 
tion Leo.  It  was  found  by  historical  research  on  the 
subject  that  this  shower  had  recurred  at  intervals  of 
about  one  third  of  a  century  for  at  least  thirteen  hundred 


280      COMETS  AND  METEORIC  BODIES 

years.     The  earliest  account  is  the  following  from  an 
Arabian  writer : 

In  the  year  599,  on  the  last  day  of  Moharren,  stars  shot  hither 
and  thither,  and  flew  against  each  other  like  a  swarm  of  locusts ; 
people  were  thrown  into  consternation  and  made  supplication  to 
the  Most  High;  there  was  never  the  like  seen  except  on  the 
coming  of  the  messenger  of  God ;  on  whom  be  benediction  and 
peace. 

The  first  well-described  shower  of  this  class  occurred  on 
November  12,  1799.  It  was  seen  by  Humboldt,  then  on 
the  Andes.  He  seems  to  have  considered  it  as  a  very  re- 
markable display,  but  made  no  exact  investigation  as  to 
its  cause. 

The  next  recurrence  was  in  1833,  which  seems  to  have 
been  the  most  remarkable  one  ever  observed.  The  as- 
tronomer Olbers  suggested  from  this  that  the  shower  had 
a  period  of  thirty-four  years,  and  predicted  a  possible 
return  in  1867,  which  actually  appeared  in  1866.  In 
1866  and  1867  the  observations  were  more  carefully 
made  than  ever  before,  and  led  to  the  remarkable  astro- 
nomical discovery,  just  alluded  to,  that  of  the  relation 
between  meteors  and  comets.  To  explain  this  we  must 
define  the  radiant  point  of  meteors. 

It  is  found  that  if ,  during  a  meteoric  shower,  we  mark 
the  course  of  each  meteor  by  a  line  on  the  celestial  sphere, 
and  continue  these  lines  backward,  we  shall  find  them  all 
to  meet  at  a  certain  point  in  the  heavens.  In  the  case 
of  the  November  meteors  this  point  is  in  the  constellation 
Leo ;  in  the  August  meteors  it  is  in  Perseus.  It  is  called 
the  radiant  point  of  the  shower.  The  lines  in  which  the 


COMETS    AND    METEORS  281 

meteors  move  are  the  same  as  if  they  were  all  shot  out 
from  this  one  point,  but  it  must  not  be  supposed  that  the 
meteors  are  actually  seen  at  this  point;  they  may  begin 
to  show  themselves  at  any  distance  from  it  less  than 
ninety  degrees ;  but  when  they  are  seen  they  are  moving 
from  the  point.  This  shows  that  the  meteors  are  all  mov- 
ing in  parallel  lines  when  they  encounter  our  atmosphere. 
The  radiant  point  is  what,  in  perspective,  is  called  the 
vanishing  point. 

Connection  of  Comets  and  Meteors 

The  period  of  the  November  meteors,  thirty-three 
years,  being  known,  and  the  exact  position  of  the  radiant 
point  determined,  it  became  possible  to  calculate  the  orbit 
of  these  objects.  This  was  done  by  Leverrier  soon  after 
the  shower  of  1866.  Now  it  happened  that,  in  December, 
1865,  a  comet  appeared  which  passed  its  perihelion  in 
January,  1866.  Careful  study  of  its  motion  showed  that 
its  period  was  about  thirty-three  years.  This  orbit  was 
computed  by  Oppolzer,  who  published  it  without  noticing 
its  resemblance  to  that  of  the  meteors.  Then  it  was  no- 
ticed by  Schiaparelli  that  there  was  an  almost  perfect  re- 
semblance between  the  orbit  of  Oppolzer's  comet  and  the 
Leverrier  orbit  of  the  November  meteors.  So  near  to- 
gether were  they  that  no  doubt  could  be  felt  that  the  two 
orbits  were  identical.  The  evident  fact  was  that  the 
bodies  which  produced  these  November  meteors  were  fol- 
lowing the  comet  in  its  orbit.  It  was  therefore  concluded 
that  these  objects  had  originally  formed  part  of  the 
comet  and  had  gradually  separated  from  it.  When  a 


283      COMETS  AND  METEORIC  BODIES 

comet  is  disintegrated  in  the  manner  described  in  the  last 
chapter,  those  portions  of  its  mass  which  are  not  com- 
pletely dissipated  continue  to  revolve  around  the  sun  as 
minute  particles,  which  get  gradually  separated  from 
each  other  in  consequence  of  there  being  no  sufficient  bond 
of  attraction,  but  they  still  follow  each  other  in  line  in 
nearly  the  same  orbit. 

•  /The  sajne  thing  was  found  to  be  true  of  the  August 
meteors.  They  are  found  to  move  in  an  orbit  very  near 
to  that  of  a  comet  observed  in  1862.  The  period  of  this 
comet  could  not  be  exactly  determined,  but  it  is  supposed 
to  be  between  one  and  two  hundred  years. 

The  third  remarkable  case  of  this  kind  occurred  in 
1872.  We  have  already  spoken  of  the  disappearance  of 
Biela's  comet.  It  happens  that  the  orbit  of  this  body 
nearly  intersected  that  of  the  earth  at  the  point  which 
the  latter  passes  toward  the  end  of  November.  From  the 
observed  period  of  this  comet  it  should  have  passed  this 
point  about  the  first  of  September,  1872,  between  two 
and  three  months  before  the  passage  of  the  earth 
through  the  same  point.  From  the  analogy  of  the  other 
cases  it  was  therefore  judged  that  there  would  be  a 
meteoric  shower  on  the  evening  of  November  27,  1872, 
and  that  the  radiant  point  would  be  in  the  constellation 
Andromeda.  This  prediction  was  fulfilled  in  every  re- 
spect. The  Andromedes,  as  these  meteors  are  called,  now 
recur  with  great  regularity. 

There  are  now  some  disappointing  circumstances  to 
narrate.  The  comet  of  1866  should  have  reappeared 
sometime  during  the  years  1898-1900,  but  it  was  not 


COMETS    AND    METEORS  283 

seen.  Probably  it  was  mfssed,  not  because  of  its  com- 
plete disintegration,  but  because  it  happened  -to  pass  its 
perihelion  at  a  time  when  the  earth  was  too  far  away  to 
admit  of  the  comet  being  visible.  But,  what  is  still  more 
curious  is  that  the  meteors  themselves,  a  shower  of  which 
was  expected  in  1899-1900,  did  not  reappear  in  great 
numbers  at  either  date.  The  probable  reason  for  this  is 
that  the  swarm  was  deflected  from  its  course  by  the  at- 
traction of  the  planets,  which  continually  changes  the 
orbit  of  every  object  of  this  kind. 

The  general  conclusion  is  that  the  countless  thousands 
of  comets  which  in  time  past  have  coursed  Around  the 
sun,  leave  behind  minute  fragments  of  their  mass,  which 
follow  in  their  orbits  like  stragglers  from  an  army,  and 
that,  when  the  earth  encounters  a  swarm  of  these  frag- 
ments a  meteoric  shower  is  produced.  But  it  is  still  an 
open  question  whether  all  these  meteoric  particles  can 
be  fragments  of  comets,  with  the  probabilities  in  favor 
of  a  negative  answer.  If  we  are  to  accept  the  conclu- 
sions drawn-  by  Professor  Elkin  from  recent  photographs 
of  meteors,  the  velocities  of  these  bodies  sometimes  exceed 
•the  parabolic  limit  described  in  the  last  chapter.  If  this 
be  so,  they  must  be  wanderers  through  the  infinite  stellar 

-spaces,  having  no  connection  with  our  system* 

• 

The  Zodiacal  Light 

This  is  a  very  soft,  faint  light,  surrounding  the  sun, 
extending  out  to  about  the  orbit-  of  the  earth,  and  lying 
nearly  in  the  plane  of  the  ecliptic.  In  tropical  latitudes 
it  may  be  seen  on  any  clear  evening  about  an  hour  or 


COMETS  AND  METEORIC  BODIES 

less  after  sunset.  In  our  latitudes  it  is  best  seen  in 
the  spring,  when,  about  an  hour  and  a  half  after  sunset, 
it  may  always  be  seen  in  the  west  and  southwest,  extend- 
ing upward  toward  the  Pleiades.  It  is  best  seen  at  this 


FlG.  49. — Tlie  Zodiacal  Light  in  February  and  March. 

season  because,  lying  in  the  plane  of  the  ecliptic,  it  makes 
a  greater  angle  with  the  horizon  then  than  at  other  sea- 
sons. In  autumn  it  may  be  seen  in  the  morning  before 
daybreak,  rising  from  the  east  and  extending  toward 
the  south. 

It  is  said  that  in  regions  where  the  atmosphere  is 
clearer  thaii  with  us,  it  may  be  seen  all  night,  spanning 
the  heavens  like  a  complete  circle.  If  so,  the  light  is  so 


THE    ZODIACAL    LIGHT  285 

faint  as  to  elude  ordinary  vision,  and  this  continuity  does 
not  seem  to  be  well  established. 

But  there  is  associated  with  it  a  phenomena  which  is 
still  one  of  the  mysteries  of  astronomy.  In  the  heavens, 
immediately  opposite  the  sun,  there  is  always  a  faint 
light,  to  which  the  term  Gegenschein  is  applied.  This  is 
a  German  word,  of  which  the  best  English  equivalent  is 
counter-glow.  The  light  is  so  faint  that  it  can  be  seen 
only  under  the  most  favourable  conditions.  When  it  falls 
in  the  Milky  Way  the  light  of  that  body  is  sufficient  to 
drown  it  out,  as  is  that  of  the  moon,  if  the  latter  is  above 
the  horizon. 

It  passes  through  the  Milky  Way  in  June  and  Decem- 
ber of  each  year,  and  can  therefore  not  be  seen  during 
these  months.  Nor  is  it  likely  to  be  seen  during  the  first 
part  of  January  or  July.  At  other  times  it  must  be 
looked  for  when  the  sun  is  considerably  below  the  horizon, 
the  sky  perfectly  clear  and  the  moon  not  in  sight.  It 
may  then  be  seen  as  an  extremely  faint  impression  of 
light,  to  which  no  exact  outline  can  be  assigned.  The 
observer  will  find  it  by  sweeping  his  eye  over  the  region 
of  the  spot  exactly  opposite  the  sun. 

There  can  be  little  doubt  that  the  zodiacal  light  is 
caused  by  the  reflection  of  the  light  of  the  sun  from  a 
swarm  of  very  minute  bodies,  perhaps  in  the  nature  of 
meteors,  continually  revolving  around  it.  We  might 
naturally  attribute  the  Gegenschein  to  the  same  cause, 
but  the  question  would  then  arise  why  it  is  only  seen 
opposite  the  sun.  It  has  been  suggested  that  possibly 
the  earth  has  a  tail,  like  a  comet,  and  that  the  Gegen- 


286      COMETS  AND  METEORIC  BODIES 

schein  is  simply  this  tail  seen  endwise.  This  is  not  an 
impossibility,  but  there  is  no  proof  that  it  is  true. 

.:":'.•  !  '  '• 

The  Impulsion  of  Light 

Facts  are  now  being  discovered,  and  physical  theories 
developed,  the  ultimate  outcome  of  which  may  be  an  ex- 
planation of  a  number  of  mysterious  phenomena  asso- 
ciated with  the  earth  and  the  universe.  These  phenomena 
are  presented  by  the  corona  of  the  sun,  the  tails  of  comets, 
the  .aurora,  terrestrial  magnetism  and  its  variations, 
nebulae,  the  Gegenschein,  and  the  zodiacal  light.  The 
theories  in  question  belong  rather  to  the  physicist  than 
the  astronomer,  and  the  writer  docs  not  feel  competent  to 
explain  them  fully  in  their  latest  form,  nor  to  define 
where  established  facts  end  and  speculation  begins.  He 
must  therefore  limit  himself  to  a  few  points. 

First  in  order  we  have  a  pressure  exerted  by  light, 
which  was  pointed  out  by  Maxwell  thirty  years  ago,  but 
which  seems  to  have  been  very  generally  overlooked,  by 
astronomers  at  least.  This  principle  was  deduced  by 
Maxwell  from  the  electro-magnetic  theory  of  light,  and 
may  be  stated  as  follows : 

;  When  a  pencil  of  light  impinges  perpendicularly  on 
an  opaque  ob j  ect,  it  produces  a  pressure  upon  the  surface 
of  that  object,  determined  by  the  condition  that  if  the 
object  were  set  in  motion  with  the  velocity  of  light,  and 
the  force  against  it  were  kept  up,  the  power  required  to 
keep  up  the  pressure  would  be  equal  to  that  carried  by 
the  ray  of  light. 

Another  way  of  expressing  Jhe  principle  is  .this:  Sup- 


THE    IMPULSION    OF    LIGHT  287 

posing  the  rays  of  light  to  be  parallel,  the  work  done  by 
the  pressure  upon  a  surface  moving  through  any  length 
of  the  pencil  is  equal  to  the  energy  of  the  light  conr 
tained  in  that  length. 

By  the  aid  of  this  principle  and  a  knowledge  of  the 
heat  or  energy  contained  in  the  rays  of  the  sun,  it  is 
possible  to  calculate  the  pressure  in  question.  It  is  found 
to  be  too  slight  to  be  detected  by  any  ordinary  mode  of 
measurement.  The  great  difficulty  arises  from  the  fact 
that,  if  the  experiment  is  not  tried  in  a  vacuum,  the  pres- 
sure will  be  confused  with  that  exerted  by  the  surround- 
ing air.  A  vacuum  so  nearly  perfect  that  the  slight 
residuum  of  air  still  contained  within  it  shall  not  exert  a 
force  comparable  with  the  light  has  not  yet  been  at- 
tained. Our  conclusion  must  therefore  depend  on  obser- 
vations made  on  minute  particles  contained  in  the  celes- 
tial spaces;  and  we  cannot  ascend  into  these  spaces  to 
make  the  experiments,  nor  can  we  send  matter  up  there 
to  be  experimented  upon.  All  we  can  do  is  to  observe 
matter  already  at  hand.  Here,  then,  is  a  wide  gap  which 
we  cannot  bridge  over  in  practice. 

The  other  element  in  the  case  is  the  discovery  that  par- 
ticles smaller  than  atoms,  called  corpuscles  or  ions,  are 
thrown  off  with  high  velocity  from  intensely  heated 
bodies.  The  sun  being  such  a  body,  it  follows  that  such 
ions  must  be  shot  out  from  it. 

On  Maxwell's  theory,  the  explanation  of  a  comet's  tail 
is  simple  in  the  extreme.  Being  in  the  vacuum  of  celes- 
tial space,  the  matter  of  the  comet  evaporates  on  the 
side  next  to  the  sun,  and,  there  being  no  pressure  to  bin- 


288      COMETS  AND  METEORIC  BODIES 

der  its  expansion,  it  begins  by  flying  off  in  all  directions, 
especially  toward  the  sun.  It  condenses  into  very  minute 
particles,  which  are  acted  upon  by  the  sun's  rays  and 
thus  thrown  in  the  direction  away  from  the  sun.  That 
the  tail  of  the  comet  was  produced  by  a  repulsion  like  this 
has  been  evident  ever  since  observations  were  made,  but 
not  until  Maxwell's  law  was  understood  could  any  ex- 
planation be  given  of  the  seeming  repulsion  of  the  matter 
of  the  tail  by  the  sun. 

The  explanations  of  the  other  phenomena  we  have  men- 
tioned are  not  yet  so  simple  and  satisfactory  that  they 
may  be  clearly  stated  in  a  short  space.  The  reader  who 
is  interested  in  the  subject  must  therefore  be  referred  to 
special  papers  and  treatises.* 

*The  papers  to  which  the  present  writer  is  principally  indebted  for  the 
views  in  question  are  by  Prof.  J.  J.  Thompson,  in  the  Popular  Science  Monthly 
for  August,  1901,  and  to  the  article  by  Prof.  John  Cox  in  the  number  for  Jan- 
uary, 1902.  These  papers  again  set  forth  the  investigations  of  Arrhenius,  the 
Swedish  physicist,  who  seem«  to  have  made  the  most  successful  endeavour 
to  explain  the  phenomena  in  question  on  the  principles  which  we  have 
(Mentioned. 


PART  VI 
THE  FIXED  STARS 


I 

GENERAL   REVIEW 

HAVING  completed  our  survey  of  that  small  section  of 
the  universe  in  which  we  have  our  dwelling,  our  next  task 
is  to  fly  in  imagination  to  those  distant  parts  of  space 
occupied  by  the  thousands  of  stars  which  stud  our  sky. 
This  is  the  field  of  astronomy  in  which  the  most  wonder- 
ful discoveries  have  been  made  in  recent  times.  We  now 
know  things  about  many  stars  which,  even  to  such  an 
observer  as  Sir  William  Herschel,  would  have  seemed  far 
beyond  the  possibilities  of  human  ken.  But  the  very 
vastness  of  the  field  and  the  minuteness  of  the  details 
into  which  recent  research  has  gone  render  it  impossible 
to  undertake  anything  like  a  comprehensive  survey  within 
the  limits  of  the  present  little  bcok.  All  we  can  do  is  to 
point  out  the  more  salient  features  of  the  universe  of  stars 
as  they  have  been  brought  to  light  by  observers  and  in- 
*  vestigators  of  the  past  and  present.  The  reader  who 
desires  further  details  and  a  wider  idea  of  the  methods 
and  results  of  recent  research  relating  to  the  stars  may 
find  them  in  a  volume  which  the  present  author  has  re- 
cently devoted  to  the  subject.* 

From  the  childhood  of  the  race  men  have  inquired: 
"What  is  a  star?"     To  this  question  no  answer  was  pos- 

*  The  Stars,  a  Study  of  the  Universe.    G.  P.  Putnam's  Sons,  New  York. 


292  THE    FIXED    STARS 

sible  until  recent  times.  Even  within  the  last  century 
little  more  could  be  said  than  that  they  were  shining 
bodies  whose  nature  was  to  us  a  mystery.  At  the  present 
time  we  may  define  the  stars  as  immense  globes  of  matter, 
generally  millions  of  times  the  size  of  the  earth,  so  in- 
tensely hot  that  they  shine  by  their  own  light,  and  so 
massive  that  they  may  continue  to  give  light  and  heat  for 
unknown  millions  of  years  without  cooling  off.  What 
we  have  said  of  the  sun  probably  applies  in  a  greater  or 
less  degree  to  the  great  majority  of  the  stars.  It  is  true 
that  we  cannot  study  their  surfaces  because,  even  in  the 
most  powerful  telescopes,  they  appear  as  mere  points  of 
light.  But  the  analogy  with  our  sun  and  with  other 
heavenly  bodies  leads  us  to  believe  that  each  of  them  re- 
volves on  its  axis  as  the  sun  docs,  and  that,  could  we  see 
it  at  the  proper  distance,  it  would  present  much  the  same 
appearance  as  our  sun.  We  have  abundant  evidence  that 
rotation  is  the  order  of  nature  in  the  case  of  all  the  heav- 
enly bodies.  In  the  few  cases  where  it  is  possible  to 
decide  whether  a  star  does  or  does  not  rotate,  the  question 
has  been  answered  in  the  affirmative. 

There  are  innumerable  differences  of  detail  among  the 
star:.  Indeed  it  would  seem  that  no  two  are  exactly  alike 
in  their  physical  constitution,  any  more  than  two  men 
are  alike  in  their  personal  appearance  and  make-up.  In 
the  chapter  on  the  sun  we  tried  to  give  an  idea  of  the 
enormous  temperature  of  that  body,  which  far  exceeds 
any  degree  of  heat  we  can  produce  on  the  earth.  We 
have  good  reason  to  believe  that,  while  the  stars  differ 
widely  in  temperature,  the  great  majority  of  them  are 


STARS    AND    NEBULAE 

far  hotter  even  than  the  sun.     This  is  true  of  their  sur- 
faces and  must  be  still  more  true  of  their  vast  interiors. 

Stars  and  Nebulce 

Stars  are  not  the  only  bodies  which  fill  these  distant 
regions  of  space.  Scattered  over  the  sky  are  immense 
masses  of  exceedingly  rare  matter  which,  from  their 
cloud-like  appearance,  are  called  nebulae.  In  size  these 
bodies  far  exceed  the  sun  or  stars.  A  nebula  only  as 
large  as  our  solar  system  would  probably  be  invisible  in 
the  most  powerful  telescope,  and  could  never  be  impressed 
even  on  the  most  delicate  photograph  of  the  sky  unless 
above  the  ordinary  brightness.  Those  that  we  know  have 
probably  hundreds  or  thousands  of  times  the  extent  of 
our  whole  solar  system.  We  may  therefore  classify  those 
bodies  of  the  universe  which  shine  by  their  own  light  as 
stars  and  nebulae. 

Spectra  of  the  Stars 

When  we  read  of  astronomical  discoveries,  we  common- 
ly think  of  them  as  being  made  by  looking  through  a 
•telescope.  But  this  13  no  longer  the  case.  The  greatest 
astronomical  development  of  recent  times  consists  in 
proving  the  existence  of  dark  bodies  of  the  nature  of 
planets,  revolving  around  many  stars.  These  objects 
are  absolutely  invisible  in  any  telescope  which  it  would 
be  possible  to  construct.  Such  an  instrument  could  tell 
us  nothing  about  the  constitution  of  a  star.  The  great 
engine  of  progress  has  been  the  spectroscope,  which  is 
described  in  a  previous  chapter.  From  what  has  there 


294  THE    FIXED    STARS 

been  said  the  reader  will  see  that,  using  words  in  their 
ordinary  sense,  we  do  not  see  anything  by  the  aid  of  a 
spectroscope.  What  we  do  with  it  is  to  analyse  the  rays 
of  light  into  their  component  parts,  just  as  a  chemist 
analyses  a  compound  body  into  its  simple  elements.  A 
spectroscopic  analysis  is  more  complicated  from  the  fact 
that  the  number  of  elements  which  compose  a  ray  of  light 
is  generally  indefinite.  The  great  advantage  of  spectro- 
scopic analysis  arises  from  the  fact  that  it  is  independent 
of  distance.  The  farther  a  star  is  away,  the  more  diffi- 
cult it  is  to  see,  whether  we  look  at  it  with  the  naked  eye 
or  through  a  telescope.  Its  light  diminishes  as  the  square 
of  the  distance  increases;  twice  as  far  away  it  gives  us 
only  one  fourth  the  light ;  three  times  as  far  away,  only 
one  ninth  the  light,  and  so  on.  But  if  enough  light  comes 
from  the  star  to  enable  its  spectrum  to  be  analysed,  the 
result  can  be  reached  equally  well  no  matter  how  great 
the  distance.  As  the  chemist  could  analyse  a  mineral 
brought  from  the  planet  Mars,  were  such  a  thing  pos- 
sible, as  easily  as  he  could  if  he  found  it  on  the  earth, 
so,  when  a  ray  of  light  reaches  the  spectroscope,  the  fact 
that  it  may  have  been  hundreds  of  years  on  its  way,  does 
not  interfere  with  the  drawing  of  conclusions  from  it. 

When  the  spectrum  of  a  star  is  formed  it  is  always 
found  to  be  crossed  by  numerous  dark  lines.  This  shows 
that  all  the  stars,  like  our  sun,  are  surrounded  by  atmos- 
pheres which  are  not  as  hot  as  the  central  body.  But  this 
does  not  imply  that  the  atmosphere  is  cold.  On  the  con- 
trary, it  is  probably  hotter  than  the  flame  of  any  furnace 
we  have  on  earth,  even  in  the  case  of  the  cooler  stars. 


SPECTRA    OF    THE    STARS  295 

When  the  spectra  of  stars  are  carefully  compared,  it  is 
always  found  that  hardly  any  two  are  exactly  alike. 
This  shows  that  their  atmospheres  all  differ  in  their  phys- 
ical constitution,  or  in  the  temperature  of  the  substances 
which  compose  them.  A  great  number  of  the  dark  lines 
of  their  spectra  are  found  to  be  identical  with  those  pro- 
duced by  known  substances  on  earth.  This  shows  that 
the  substances  of  which  the  stars  are  made  up  are  iden- 
tical, in  at  least  a  great  part,  with  those  on  the  earth. 

One  of  the  most  abundant  of  these  substances  is  hydro- 
gen. Several  lines  of  hydrogen  are  found  in  nearly  all 
the  stars.  Another  substance  which  seems  to  be  almost 
universal  throughout  the  universe  is  iron.  Yet  another  is 
calcium,  the  metallic  base  of  lime.  We  all  know  that  this 
substance  abounds  on  the  earth,  and  we  have,  in  its  diffu- 
sion among  the  stars,  an  example  of  the  unity  of  nature 
in  its  widest  extent. 

Yet,  variety  is  also  the  rule.  Besides  lines  due  to  known 
substances,  many  stars  show  lines  which  have  not  yet  been 
identified  with  those  of  any  element  that  we  know  of. 
This  is  especially  the  case  in  the  class  known  as  Orion 
'stars,  because  many  of  them  are  found  in  the  constella- 
tion Orion.  These  stars  are  mostly  very  white  or  even 
blue  in  colour,  and  show  a  number  of  fine  dark  lines  which 
are  to  a  greater  or  less  extent  the  same  in  all  Orion  stars, 
but  are  not  those  produced  by  any  known  chemical 
element.  We  therefore  have  reason  to  believe  that  there 
are  in  the  stars  other  chemical  elements  than  those  with 
which  we  are  acquainted. 

There  is  a  very  curious  case  in  which  an  element  first 


296  THE    FIXED    STARS 

excited  interest  through  its  being  found  in  the  sun  and 
stars.  For  some  time  after  the  study  of  the  sun's  spec- 
trum had  been  commenced,  it  was  known  that  certain  well- 
marked  lines  in  it  were  not  produced  by  any  substance 
then  known.  But  continued  research  led  to  the  discovery 
that  this  substance  existed  in  a  Norwegian  mineral, 
cleveite,  and  perhaps  elsewhere  en  the  earth.  From  its 
existence  on  the  sun  it  was  called  helium.  Its  spectrum 
was  no  sooner  made  known  than  it  was  found  that  helium 
existed  in  many  stars  which  are,  fcr  that  reason,  called 
"helium  stars." 

Density  -and  Heat  of  the  Stars 

In  many  cases  some  idea  can  be  obtained  of  the 
density  of  a  star,  or,  in  ordinary  language,  of  its 
specific  gravity.  It  is  very  remarkable  that,  in  near- 
ly all  such  cases  the  density  is  found  to  be  far  less 
than  that  of  our  ordinary  solid  or  liquid  substances; 
frequently  no  greater  than  that  of  air,  sometimes 
even  less.  In  this  respect  our  sun,  although  its  den- 
sity is  so  small,  seems  to  be  an  exception,  and  it  is 
likely  that  only  a  very  small  proportion  of  the  stars  are 
as  dense  as  the  sun.  This  affords  one  proof  of  the  high 
temperature  of  these  bodies,  which  must  be  such  that  all 
liquid  or  solid  substances  exposed  to  it  would  boil  away 
as  water  bcils  when  put  on  a  fire,  thus  changing 
its  substance  into  a  vapour.  We  have  reason  to  be- 
lieve that  the  stars  are  for  the  most  part  masses  of 
this  intensely  hot  vapour,  surrounded  perhaps  by  a 
somewhat  colder  surface.  Possibly  many  of  the  stars 


DENSITY  AND  HEAT  OF  THE  STARS  297 

may  be  of  the  nature  of  bubbles,  but  this  is  far  from 
being  established. 

A  star,  like  the  sun,  must  be  hotter  in  the  interior  than 
at  its  surface.  From  the  latter  alone  can  heat  be  radiated ; 
hence  the  surface  is  continually  cooling  off,  and  if  the 
matter  composing  the  body  were  at  rest,  the  cooling 
would  soon  go  so  far  that  a  crust  would  form,  as  it  does 
on  a  mass  of  molten  iron.  The  only  way  in  which  this 
can  be  prevented  is  that,  as  the  superficial  portions  cool, 
the  greater  density  which  they  thus  acquire  causes  them 
to  sink  down  into  the  seething  mass  below,  portions  of 
which  arise  to  take  their  place,  cool  off,  and  sink  in  their 
turn.  Thus  there  is  a  continual  interchange  of  matter 
between  the  inside  and  the  surface,  much  as  in  a  boiling 
pot  the  water  at  the  bottom  is  continually  being  forced 
up  to  the  top,  while  that  on  the  top  continually  sinks 
down. 

It  follows  from  this  that  there  must  be  a  limit  to  the 
smallness  of  a  star.  If  such  a  body  were  no  larger  than 
the  moon,  it  would,  in  a  few  thousand  years,  so  far  cool 
off  that  a  crust  would  form  over  its  surface.  This  would 
•cut  off  the  currents  by  wrhich  the  hot  matter  is  brought 
to  the  surface  and  the  star  would  soon  cease  to  shine.  As 
there  can  be  little  doubt  that  the  age  of  most  of  the  stars 
is  to  be  reckoned  by  millions  of  years,  it  follows  that  they 
must  be  so  large  that  they  can  lose  heat  for  millions  of 
years  and  yet  a  cool  crust  not  form  on  their  surface. 

We  have  said  that  our  sun  is  among  the  colder  of  the 
stars  and  also  that  it  is  among  the  smaller.  These  two 
facts  fit  well  together.  The  smaller  a  star  is  the  mor§ 


298  THE    FIXED    STARS 

rapidly  it  cools  off,  just  as  a  cup  of  water  cools  off  faster 
than  a  pot  full. 

The  revelations  of  the  spectroscope  makes  it  very  prob- 
able that  every  star  has  a  life  history.  It  began  as  a 
nebula,  which,  in  the  course  of  ages,  slowly  condensed 
into  an  intensely  hot,  blue-coloured  star.  The  condensa- 
tion going  on,  the  star  becomes  yet  hotter,  until  it  reaches 
its  highest  temperature.  Then,  cooling  off,  its  colour 
changes  to  white,  yellow,  and  red,  and  the  lines  in  its 
spectrum  become  darker  and  more  numerous.  Finally,  its 
light  dies  away,  as  a  fire  flickers  out  when  the  supply  of 
fuel  is  exhausted,  and  the  star  becomes  a  dark  opaque 
body, — its  life  has  ended.  The  greater  the  mass  of  the 
star  the  longer  its  life.  Thus  it  is  that  the  stars  we 
observe  seem  to  be  of  all  ages,  from  the  infantile  nebula 
to  the  star  dying  of  old  age. 


n 

ASPECT  OF  THE  SKY 

NOT  only  to  the  ordinary  beholder,  but  to  the  learned 
student  of  the  heavens,  the  most  wonderful  feature  of 
the  sky  is  the  Milky  Way.  This  is  a  girdle  apparently 
spanning  the  sky  and  perhaps,  in  reality,  spanning  the 
entire  universe  of  stars,  uniting  them,  as  it  were,  into  a 
single  system — one  "stupendous  whole."  It  may  be  seen 
at  some  time  of  the  night  every  day  of  the  year,  and  at 
some  convenient  hour  in  the  evening  of  every  month  ex- 
cept May.  During  this  month  it  extends  round  the 
horizon  in  the  early  evening,  and  is  invisible  through 
the  denser  strata  of  the  air.  Of  course  it  will  even 
then  become  visible  in  the  east  and  northeast  later 
at  night. 

The  smallest  telescope  will  show  the  Milky  Way  to  be 
formed  of  immense  congeries  of  stars,  too  faint  in  their 
.light  to  be  separately  visible  at  their  great  distance  from 
us.  Careful  observation,  even  with  the  naked  eye,  will 
show  that  these  stars  are  not  equally  scattered  along  the 
whole  extent  of  their  course,  but  are  frequently  collected 
in  great  masses  or  clusters,  with  comparatively  empty 
spaces  around  or  between  them.  These  are  especially 
marked  in  the  portions  of  the  belt  visible  in  the  south  in 
the  evenings  of  summer  and  autumn. 

A  remarkable  fact  connected  with  the  universe  is  that 


300  THE    FIXED    STARS 

the  stars  are  not  equally  thick  in  all  directions,  there  be- 
ing more  in  a  given  space  around  the  belt  of  the  Milky 
Way,  and  the  number  growing  smaller  as  we  pass  away 
from  that  belt.  This  is  true  even  of  the  brightest  stars, 
and  yet  more  true  of  the  fainter  ones.  The  poles  of  the 
Milky  Way  are  those  two  points  in  the  heavens  which 
are  ninety  degrees  from  every  point  of  the  Milky  Way. 
If  we  imagine  one  to  hold  a  rod  in  his  hand,  so  that  the 
Milky  Way  shall  be  at  right  angles  to  it,  the  two  ends  of 
the  rod  will  point  to  the  two  poles  in  question.  To  give 
an  idea  of  the  thickness  of  the  stars  we  may  say  that, 
near  the  poles  of  the  Milky  Way,  a  round  circle  of  the 
sky  one  degree  in  diameter  will  commonly  contain  two 
or  three  stars  visible  in  quite  a  small  sized  telescope.  In 
the  region  of  the  Milky  Way,  such  a  circle  may  contain 
eight,  ten,  perhaps  even  fifteen  or  twenty  such  stars. 

Brightness  of  the  Stars 

No  one  can  look  at  the  sky  without  seeing  that  the 
stars  differ  enormously  in  their  brightness,  or,  in  the 
language  of  astronomy,  in  their  magnitude.  They  re- 
semble men  in  that  a  very  few  far  outshine  all  their  fel- 
lows, a  greater  number  are  less  bright,  and,  as  we  come 
down  to  smaller  and  smaller  stars,  we  find  the  number 
to  continually  increase.  Those  visible  to  the  naked  eye 
were  classified  by  the  ancient  astronomers  as  of  six  orders 
of  magnitude.  About  twenty  of  the  brightest  in  the  sky 
were  designated  as  of  the  first  magnitude.  The  forty 
next  in  order  of  brightness  were  called  of  the  second  mag- 
nitude; a  larger  number  were  of  the  third,  and  so  on  to 


BRIGHTNESS    OF    THE    STARS         301 

the  sixth  magnitude,  which  included  the  faintest  stars 
that  the  best  eye  could  see  under  a  clear  sky. 

Modern  astronomers  carry  this  system  down  to  the 
telescopic  stars.  Those  which  are  one  degree  fainter  than 
the  smallest  visible  to  the  naked  eye  are  called  of  the 
seventh  magnitude;  the  next  in  brightness  are  of  the 
eighth,  and  so  on.  The  faintest  that  can  be  seen  or 
photographed  with  the  largest  telescopes  are  probably 
of  the  fifteenth,  sixteenth,  or  seventeenth  magnitude. 

The  reader  will  of  course  understand  that  the  magni- 
tude of  a  star  does  not  express  its  real  brightness,  because 
a  shining  body  looks  brighter  the  nearer  it  is  to  us.  No 
matter  how  bright  a  star  may  be,  if  it  were  removed  far 
enough  away  it  would  grow  so  faint  as  to  be  invisible. 
The  smallest  star  in  the  heavens  if  brought  near  enough 
to  us  would  shine  as  of  the  first  magnitude. 

It  was  formerly  believed  that  the  actual  brightness  of 
the  different  stars  was  nearly  the  same,  and  that  some 
looked  brighter  than  others  only  because  they  were 
nearer  to  us.  But  the  case  is  now  known  to  be  different. 
Estimates  of  the  distance  of  the  stars  show  that  among 
•the  nearest  to  us  are  many  quite  invisible  to  the  naked 
eye,  while  some  of  the  first  magnitude  are  so  far  away 
that  their  distance  is  immeasurable.  The  brightest  ones 
probably  emit  hundreds  of  thousands  of  times  as  much 
light  as  the  smallest  ones. 

Number  of  Stars 

The  whole  number  of  stars  in  the  heavens  which  can 
be  seen  by  the  ordinary  eye  is  between  five  and  six  thou- 


THE    FIXED    STARS 

sand.  Possibly  a  very  keen  eye  might  see  more  than  six 
thousand,  but  most  eyes  will  see  even  less  than  five  thou- 
sand. Of  these  only  one  half  can  be  above  the  horizon  at 
the  same  time,  and  of  this  half  a  great  number  will  be 
so  near  the  horizon  as  to  be  obscured  by  the  great  thick- 
ness of  the  atmosphere  in  that  direction.  The  number 
which  can  readily  be  seen  on  a  clear  evening  by  an 
ordinarily  good  eye  will  probably  range  between  fifteen 
hundred  and  two  thousand.  Stars  visible  to  the  naked 
eye  are  called  lucid  stars,  to  distinguish  them  from  tele- 
scopic stars,  which  can  be  seen  only  by  the  aid  of  a 
telescope. 

It  is  impossible  to  make  even  an  estimate  of  the  total 
number  of  telescopic  stars.  It  is  commonly  supposed 
that  between  fifty  and  one  hundred  million  can  be  seen 
with  large  telescopes,  and  it  is  now  possible,  with  spe- 
cially arranged  telescopes,  to  photograph  stars  which 
are  fainter  than  the  smallest  the  eye  can  see  in  any  tele- 
scope. There  is  no  sign  of  any  limit  to  the  number.  As 
we  pass  to  fainter  and  fainter  degrees  of  brightness  the 
stars  are  found  to  be  more  and  more  numerous.  All  that 
we  can  say  of  the  total  number  is  that  it  must  be  counted 
by  hundreds  of  millions. 

We  have,  in  fact,  some  reason  for  inferring  that  the 
great  majority  of  the  stars  are  invisible  in  the  most 
powerful  telescope  we  can  make,  owing  to  their  distance. 
The  distance  of  the  great  majority  is  such  that  only  the 
brightest  of  them  can  become  known  to  us. 

Minute  stars  are  here  and  there  collected  into  clusters 
in  various  parts  of  the  sky.  Some  of  these  clusters  are 


NUMBER    OF    STARS  303 

visible  to  the  naked  eye.  Those  in  and  near  the  Milky 
Way  frequently  contain  hundreds  01  even  thousands  of 
stars  too  small  to  be  seen  separately  without  a  telescope. 
The  stars  differ  from  each  other  in  colour,  although 
not  in  so  marked  a  degree  as  terrestrial  objects.  The 
most  casual  observer  cannot  fail  to  note  the  difference 
between  the  bluish  white  of  Alpha  Lyras  and  the  reddish 
light  of  Arcturus.  There  seems  to  be  a  regular  grada- 
tion in  the  colour  of  the  stars  from  blue,  through  yellow, 
to  red.  These  differences  of  colour  are  connected  with 
differences  in  the  spectra  of  the  stars.  As  a  general  rule, 
the  redder  a  star  is,  the  greater  the  number  and  intensity 
of  the  dark  lines  that  can  be  seen  in  the  green  and  blue 
parts  of  its  spectrum. 

Constellations 

A  slight  examination  of  the  heavens  shows  that  the 
stars  are  not  scattered  equally  over  the  sky,  but  that  there 
is  more  or  less  of  a  tendency  to  collect  into  constellations. 
This  is  especially  the  case  with  the  brighter  stars.  But 
no  well-marked  dividing  line  between  the  constellations  is 
possible ;  that  is,  we  cannot  draw  a  line  showing  exactly 
"where  one  constellation  ends  and  another  begins.  Never- 
theless a  division  into  constellations  was  made  in  ancient 
times  and  has  been  followed  by  astronomers  down  to  the 
present  time. 

How  and  by  whom  the  constellations  were  first  mapped 
out  and  named  no  one  knows.  The  Chinese  had  their 
asterisms — collections  of  stars  smaller  than  what  we  call 
constellations — in  the  earliest  years  of  their  history. 


304  THE    FIXED    STARS 

What  we  know  of  the  constellations  dates  from  Ptolemy, 
who  lived  in  the  second  century  after  Christ.  His  names 
are  still  in  use.  As  many  of  them  are  those  of  the  gods, 
goddesses,  and  heroes  of  Grecian  mythology — Perseus, 
Andromeda,  Cepheus,  Hercules,  etc. — it  seems  likely  that 
they  were  assigned  during  or  after  the  heroic  age. 

In  modern  times  quite  a  number  of  new  constellations 
have  been  carved  out  of  or  drawn  between  the  older  ones. 
This  is  especially  the  case  in  the  southern  hemisphere,; 
which  was  imperfectly  known  to  the  ancient  Greeks. 


m 

DESCRIPTION   OF   THE  CONSTELLATIONS 

THE  present  chapter  is  intended  for  those  who  wish  to 
be  able  to  recognise  the  principal  constellations,  and  to 
know  where  to  look  for  the  several  planets.  The  problem 
of  pointing  out  the  constellations  is  complicated  by  the 
effect  of  the  twofold  motion  of  the  earth ;  on  its  axis  and 
around  the  sun.  In  consequence  of  the  former  the  con- 
stellations change  their  apparent  position  in  the  course  of 
the  night,  and  the  result  of  the  latter  is  that  different 
constellations  are  seen  at  different  seasons. 

We  explained  in  a  former  chapter  how,  in  consequence 
of  the  motion  of  the  earth  in  its  orbit  round  the  sun,  the 
latter  seems  to  us  to  perform  an  annual  circuit  among 
the  constellations.  Hence,  if  a  star  is  east  of  the  sun,  we 
shall  see  it  approach  nearer  to  the  sun  every  day.  If  we 
look  out  night  after  night  at  the  same  hour  we  shall  find 
it  farther  and  farther  advanced  toward  the  west.  In 
consequence  of  this  change  it  must  rise  and  set  earlier 
every  day  than  it  did  the  day  before.  More  exactly,  the 
time  between  two  risings  and  settings  of  the  same  star  is 
twenty-three  hours  fifty-six  minutes  four  and  a  half  sec- 
onds. While  in  the  course  of  a  year  the  sun  rises  three 
hundred  and  sixty-five  times,  a  star  rises  three  hundred 
and  sixty-six  times.  The  latter  will  therefore  during  the 
year  have  risen  at  every  hour  of  the  day  and  night. 


306  THE    FIXED    STARS 

Astronomers  avoid  all  confusion  from  this  cause  by 
the  use  of  sidereal  time,  that  is  star-time,  or  time  meas- 
ured by  the  stars.  As  already  explained,  a  sidereal  day 
is  the  interval  between  two  successive  pasages  of  a  star 
over  the  meridian,  and  is  three  minutes  fifty-sh;  seconds 
less  than  our  ordinary  day.  It  is  divided  into  twenty- 
four  sidereal  hours,  and  each  hour  into  sidereal  minutes 
and  seconds.  A  sidereal  clock  gains  three  minutes  fifty- 
six  seconds  daily  on  an  ordinary  clock  and  thus  shows 
the  same  time  at  the  same  position  of  the  stars  the  year 
around. 

One  who  wishes  to  keep  the  run  of  the  stars  will  find  it 
very  convenient  to  have  some  idea  of  sidereal  time.  This 
may  be  had  by  the  following  rule :  Double  the  number  of 
the  month;  the  product  will  be  the  sidereal  time  at  six 
o'clock  in  the  evening.  At  seven  o'clock  it  will  be  one 
hour  later,  and  at  eight  it  will  be  two  hours  later,  and  so  on. 

Suppose,  for  example,  that  one  looks  at  the  sky  in 
November  at  nine  o'clock  in  the  evening.  This  is  the 
eleventh  month;  multiplying  by  two  gives  twenty-two, 
adding  three  gives  twenty-five,  from  which  we  drop 
twenty-four,  giving  one  hour  as  the  sidereal  time.  The 
time  thus  obtained  will  not  often  be  more  than  an  hour 
in  error,  except  during  the  first  week  or  ten  days  of  the 
month,  when  it  may  be  an  hour  or  more  too  great.  It 
may  then  be  diminished  by  one  hour. 

Applying  the  same  rule  in  January  we  have  five  hours 
as  the  sidereal  time  at  nine  in  the  evening.  But  early  in 
the  month  the  sidereal  time  at  nine  in  the  evening  will  be 
four  hours  instead  of  five. 


THE  NORTHERN  CONSTELLATIONS    307 

At  0  hours  sidereal  time  the  equinoctial  colure  is  on 
the  meridian ;  at  six  hours,  the  solstitial  colure,  and  so  on. 

The  Northern  Constellations 

With  this  preliminary  explanation  let  us  proceed  to  the 
study  of  the  constellations.  I  assume  the  reader  to  be 
somewhere  in  the  latitude  of  the  United  States.  Then 
the  principal  northern  constellations  will  never  set,  and 
will  be  visible  in  whole  or  in  part  every  evening  in  the 
year.  With  them,  therefore,  we  begin. 

A  figure,  showing  these  constellations,  is  found  in  the 
first  part  of  the  present  book  (Fig.  2).  To  see  how  they 
will  appear  hold  the  cut  with  the  month  at  top ;  we  then 
have  the  position  at  eight  o'clock  in  the  evening.  For  a 
later  hour  turn  it  a  little  in  the  direction  of  the  arrows. 
For  example,  in  July,  at  ten  o'clock,  we  hold  it  so  as  to 
have  August  at  the  top.  The  Roman  numerals  on  top 

give  the  sidereal  time 
without  the  trouble  of 
calculating  it. 

First  find  Ursa  Ma- 
jor, the  Great  Bear, 
generally  called  the  Dip- 
per, an  implement  which 
the  constellation  resem- 
FIG.  50.—  Ursa  Major,  or  TIte  Dipper.  bles  much  more  than  it 

does  a  bear.     This  you 

can  always  do  except  perhaps  in  autumn  when,  if  you 
are  far  south,  it  may  be  more  or  less  below  the  northern 
horizon.  Notice  the  pair  of  stars  forming  the  outside 


THE    FIXED    STARS 


of  the  bowl  of  the  dipper.  They  are  called  the  Pointers, 
because  they  point  toward  the  pole  star,  as  shown  by  the 

dotted  line.  This  is  the  cen- 
tral star  of  the  map.  It  is 
called  Polaris. 

The  pole  star  belongs  to 
the  constellation  Ursa  Minor, 
the  Lesser  Bear ;  the  rest  of 
Fio.&l.—  UrsaMnor.  the    constellation    you    will 

see  by   following   a   curved 

line  of  stars  from  the  pole  toward  XVI  hours.  You  will 
thus  fall  on  another  star  as  bright  as  Polaris  but  a  little 
redder  in  colour.  This  is  Beta  Ursae  Minoris. 

If  you  cannot  see  the  pointers  you  will  still  easily  find 
Polaris  if  you  know  the  exact  north,  because  it  is  nearly 
midway  between  the  zenith  and  the  northern  horizon — 
nearer   the   latter,   however, 
the  farther  south  we  are.    It 
can  be  easily   distinguished 
from    its    neighbour,    Beta, 
by   its   whiter   colour,   Beta 
being  slightly  red  or  dingy 
in  comparison. 

On  the  opposite  side  of 
the  pole,  at  the  same  dis- 
tance as  Ursa  Major,  is 

Cassiopeia,  the  Lady  in  the  Chair.  The  chair  has  a  very 
crooked  back  but  could  be  made  comfortable  by  a  cushion 
in  the  hollow. 

There  are  several  other  constellations  in  the  region 


FIG.  52. —  Cassiopeia. 


THE  AUTUMNAL  CONSTELLATIONS    309 

around  the  pole,  but  they  have  few  bright  stars  and  are 
of  less  interest  than  those  we  have  mentioned.  Among 
them  is  Draco,  the  Dragon,  whose  form  coils  itself  up  be- 
tween the  Bears,  and  whose  head  is  represented  by  a  tri- 
angle of  stars  in  XVIII  hours,  near  the  August  zenith. 

The  Autumnal  Constellations 

The  zenithal  and  southern  constellations  to  be  looked 
for  will  vary  with  the  season.  We  begin  with  the  posi- 
tion of  the  sphere  at  0  hours  sidereal  time,  which  occurs 
at  ten  o'clock  in  October,  eight  in  November,  and  six  in 
December. 

The  equinoctial  colure  is  first  to  be  imagined.  It 
passes  from  the  pole  upward  near  the  westernmost  bright 
star  of  Cassiopeia  and  can  be  traced  south  through  the 
eastern  side  of  the  square  of  Pegasus.  The  latter  easily 
recognised  landmark  of  the  sky  is  formed  by  four  stars 
of  the  second  or  third  magnitude.  The  square  is  fifteen 
degrees  on  a  side. 

Northeast  from  the  northeast  corner  of  the  square  is 
the  Great  Nebula  of  Andromeda.  It  is  plainly  visible  to 
the  naked  eye  as  a  whitish,  ill-defined  patch  of  light,  and 
is  a  fine  object  when  seen  in  a  telescope. 

The  Milky  Way  now  spans  the  heavens  like  a  slightly 
inclined  arch,  resting  on  the  east  and  west  regions  of  the 
horizon,  and  having  its  keystone  a  little  north  of  the 
zenith,  in  Cassiopeia.  Tracing  it  from  this  constellation 
toward  the  east,  we  first  have  Perseus,  which  stands  in 
the  Milky  Way  itself.  The  brightest  star  in  this  con- 
stellation is  Alpha  Persei,  of  the  second  magnitude. 


310  THE    FIXED    STARS 

East  of  Alpha  is  a  white  mass  like  a  little  cloud.  With 
a  small  telescope,  even  with  a  good  field  glass,  we  see  this 
mass  to  be  a  collection  or  cluster  of  small  stars.  It  is  the 
Great  Cluster  of  Perseus  and,  in  the  figure  of  the  con- 
stellation, forms  the  hilt  of  the  hero's  sword. 

In  a  sort  of  offshoot  toward  the  south  (or  southeast 
as  the  constellation  is  now  situated)  lies  a  row  of  three 
stars.  The  middle  and  brightest  of  these  is  the  wonder- 
ful variable  star,  Algol,  whose  changes  will  be  described 
in  a  later  chapter.  It  is  also  called  Beta  Persei. 

Below  Perseus,  the  first  large  constellation  is  Auriga, 
the  Charioteer.  It  is  marked  by  Capella,  the  Goat,  a  star 
of  the  first  magnitude  and  one  of  the  brightest  now  above 
the  horizon — indeed,  one  of  the  four  or  five  brightest  in 
the  sky.  But  it  has  no  other  striking  stars. 

In  the  southeast  are  Aldebaran  and  the  Pleiades, 
which  will  be  described  later.  Meanwhile  let  us  follow 
the  course  of  the  Milky  Way  from  the  zenith  toward 
the  west. 

The  first  collection  of  bright  stars  west  of  Cassiopeia 
is  now  Cygnus,  the  Swan,  lying  centrally  in  the  Milky 
Way.  Five  stars  are  arranged  somewhat  in  the  form  of 
a  cross  and  mark  the  body,  neck,  and  extended  wings  of 
the  bird.  The  brightest  of  the  group  is  Alpha  Cygni, 
or  Deneb,  nearly,  but. not  quite,  of  the  first  magnitude. 

Low  and  to  the  right  of  Cygnus,  and  a  little  outside  of 
the  Milky  Way,  is  the  constellation  Lyra,  the  Harp, 
marked  by  the  beautiful  and  very  bright  bluish  star, 
Vega.  It  has  no  other  star  of  greater  magnitude  than 
the  third,  but  what  it  has  will  repay  careful  study. 


•  B 


THE  AUTUMNAL  CONSTELLATIONS    3111 

In  the  figure  given  here,  notice  the  star  to  the  left  of 
Vega ;  Epsilon  Lyrse  it  is  called.  A  keen  eye  will,  on  care- 
ful examination,  see  that  this  star  is  really  composed  of 
two,  lying  so  close  together  that  it  is  not  easy  to  dis- 
tinguish them.  With  an  opera  glass  this  will  more  easily 
be  accomplished.  But  the 
most  curious  fact  is  that  if 
a  telescope  be  pointed  at 
the  pair,  each  of  the  stars 
will  be  found  to  be  double, 
so  that  Epsilon  Lyrae  is 
really  composed  of  four 
stars. 

Another  star,   about   as 
near  to  Vega  as  Epsilon  is, 
lies  at  one  corner  of  a  par- 
allelogram     or      elongated  FIG.  53.— Lyra,  the  Harp. 
diamond,    which    stretches 

south  of  Vega.  At  the  farther  blunt  corner  of  the  dia- 
mond lies  Beta  Lyrae,  marked  B  in  the  figure,  a  remark- 
able variable  star.  To  the  left  of  it  is  Gamma.  The  law 
of  variation  will  be  described  in  a  later  chapter. 

To  the  right  of  Lyra,  and  in  the  Milky  Way,  lies 
Aquila,  the  Eagle.  It  will  be  described  later. 

The  other  constellations  low  in  the  west  will  be  de- 
scribed later.  At  present  we  shall  pass  rapidly  over  the 
constellations  of  the  Zodiac. 

If  the  ecliptic  were  painted  on  the  sky  we  should  now 
see  it  rising  to  the  north  of  the  east  point  of  the  horizon, 
passing  in  the  south  to  mid-sky,  where  it  would  cross  the 


THE    FIXED    STARS 

equator  at  a  small  angle,  and  then,  passing  to  the  west, 
reach  the  western  horizon  twenty-three  degrees  south 
of  west.  At  the  time  we  suppose,  Sagittarius,  the  Archer, 
is  mostly  below  the  western  horizon.  Capricornus,  the 
Goat;  Aquarius,  the  Water  Bearer,  and  Pisces,  the 
Fishes,  fill  up  the  space  to  the  meridian.  The  stars  of 
these  constellations  are  mostly  faint,  few  or  none  exceed- 
ing the  third  magnitude. 

Reaching  the  meridian,  we  see  the  square  of  Pegasus 
above  the  Zodiac,  not  far  south  of  the  zenith.  East  of 
it  is  the  constellation  Aries,  the  Ram.  Three  of  its  prin- 
cipal stars,  of  the  second,  third,  and  fourth  magnitudes, 
form  an  obtuse  triangle.  The  brightest  is  Alpha  Arietis. 

Two  thousand  years  ago  this  constellation  marked  the 
first  sign  of  the  zodiac,  and  the  equinox  was  just  below 
Alpha  Arietis,  as  explained  in  speaking  of  the  precession 
of  the  equinoxes. 

Southeast  from  the  square  of  Pegasus  is  a  widely 
extended  constellation,  Cetus,  the  Whale.  Its  two  bright- 
est stars,  Alpha  and  Beta,  are  of  the  second  magnitude. 
The  latter  lies  nearly  below  the  southeast  star  of  the 
square  of  Pegasus  and  is  quite  by  itself.  Alpha  is  some 
distance  farther  east.  West  of  Alpha,  and  a  little  south, 
is  a  remarkable  star,  Mira  Ceti,  the  wonderful  star  of 
Cetus,  which  is  invisible  to  the  naked  eye  except  for  a 
month  or  two  in  each  year,  when  it  attains  the  fourth, 
third,  and  often  the  second  magnitude. 

A  little  west  of  south,  quite  low  down,  is  Fomalhaut, 
nearly  of  the  first  magnitude,  in  the  constellation  Pisces 
Australia,  the  Southern  Fish. 


THE    WINTER    CONSTELLATIONS      313 


The  Winter  Constellations 

The  next  position  of  the  stars  we  shall  describe  comes 
six  hours  after  the  preceding  one ;  that  is  at  two  o'clock 
A.  M.  in  November  and  at 
eight  o'clock  P.  M.  in  Feb- 
ruary. During  this  six- 
hour  interval  another  sec- 
tion of  the  Milky  Way  has 
risen  in  the  east  and  passed 
over  toward  the  south.  The 
Milky  Way  now  passes 
nearly  through  the  zenith, 
resting  on  the  horizon  near  FlG  54._77ie  HyadMm 

the  north  and  south  points. 

Near  its  course  and  east  of  the  meridian  we  see  the 
constellation  Taurus,  the  Bull,  of  which  the  brightest  star 

is  Aldebaran,  form- 
ing the  eye  of  the 
bull  in  the  mytho- 
logical figure.  Alde- 
baran is  easily  rec- 
ognised by  its  red 
colour.  It  lies  on  the 
end  of  one  branch  of 
a  V-shaped  cluster 
called  Hyades.  No- 
tice the  pretty  pair 
of  stars  in  the  middle 
of  one  leg. 


FIG.   55. — TJie  Pleiades,  as  seen  toith  the 
naktd  eye. 


THE    FIXED    STARS 

Near  by  is  the  best  known  cluster  in  the  sky,  the 
Pleiades,  or  "seven  stars."  Only  six  stars  are  made  out 
by  ordinary  unaided  vision,  but  to  a  good  eye  five  others 


• 
• 

TAYGETAj* 


ALCYONE^  ELECTRA^fc 


FIG.  56. — Telescopic  View  of  the  Pleiades,  with  Names  of  the  Brighter  Stars. 

are  visible,  making  eleven  in  all.  The  term  "seven  stars" 
is  therefore  a  misnomer ;  as  a  reason  for  it,  it  was  said  in 
ancient  times  that  the  number  was  originally  seven  but 
that  one  faded  away.  This  "lost  Pleiad"  is  probably  a 
myth,  as  we  do  not  find  stars  fading  away  permanently. 


THE    WINTER    CONSTELLATIONS      315 

With  a  telescope  we  find  the  cluster  to  contain  quite  a 
number  of  yet  smaller  stars,  as  can  be  seen  by  the  tele- 
scopic view  which  we  give. 

The  central  and  brightest  star  of  the  group  is 
called  Alcyone,  and  was  supposed  by  Maedler  to  be  the 
central  star  of  the  universe.  But  this  notion  is  quite 
baseless. 

East  of  Taurus  and  near  the  zenith  is  Gemini,  the 
Twins,  marked  by  two  stars  nearly  of  the  first  magnitude, 
Castor  and  Pollux.  The  latter  is  the  northernmost  and  a 
little  the  brighter  of  the  two. 

The  next  zodiacal  constellation  is  Cancer,  the  Crab, 
but  it  contains  no  conspicuous  stars.  Its  most  noticeable 
feature  is  Prcesepe,  a  cluster  of  stars,  which  are  singly 
invisible  to  the  naked  eye,  and  look  collectively  like  a 
small  patch  of  light.  The  smallest  telescope  will  show  a 
dozen  stars  in  the  patch. 

Leo,  the  Lion,  is  also  well  up  in  the  east.  It  may  be 
recognised  by  Regulus,  a  star  nearly  of  the  first  magni- 
tude, and  a  curved  row  of  stars  in  the  form  of  a  sickle, 
of  which  Regulus  is  the  handle. 

In  the  south  we  now  have  the  most  brilliant  constella- 
tion in  the  heavens,  the  beautiful  Orion.  The  three  stars 
of  the  second  magnitude  in  a  row  forming  the  belt  of  the 
warrior  are  familiar  from  childhood  to  all  who  watch  the 
sky.  Below  them  hangs  another  row  of  three  stars,  the 
upper  one  quite  faint.  The  middle  one  of  these  has  a 
hazy  aspect,  and  is  really  not  a  star  at  all,  but  one  of  the 
most  splendid  objects  in  the  sky,  the  Great  Nebula  of 
Orion.  A  mere  spy-glass  will  show  its  character,  but  a 


316 


THE    FIXED    STARS 


large  telescope  is  required  to  bring  out  the  magnificence 
of  its  form. 

The  corners  of  the  constellation  are  marked  by  four 
stars.  The  brighter  of  the  two  uppermost,  Alpha 

Orionis,  or  Betcl- 
guese,  is  reddish  in 
colour.  At  the  oppo- 
site corner  is  Rigel, 
blue  in  colour  and 
also  of  the  first  mag- 
nitude. The  two  up- 
per stars  are  in  the 
shoulders  of  the  fig- 
ure. Midway  and 
above  them  a  triangle 
of  small  stars  forms 
the  head. 

East  of  Oricn  is 
Cams  Minor,  the  Lit- 
tle Dog,  containing 

Procyon,  of  the  first  magnitude.  Below  it  and  south- 
east of  Orion  is  another  collection  of  bright  stars  forming 
the  constellation  Cams  Major,  the  Great  Dog,  containing 
Sirius,  the  Dog  Star,  the  brightest  fixed  star  in  the 
heavens. 

The  Spring  Constellations 

The  third  position  of  the  sphere,  sidereal  time  twelve 
hours,  occurs  in  February  at  two  A.  M. ;  in  May  at 
eight  P.  M.  Lyra  has  now  risen  in  the  northeast  and 
Capella  is  going  downward  in  the  northwest.  The  Milky 


FIG.  57. — Orion. 


THE    SPRING    CONSTELLATIONS       317 

Way  may  not  be  visible  at  all  unless  the  air  is  very  clear. 
It  will  then  be  seen  skirting  the  northern  and  western 
horizon.  Regulus  has  passed  the  meridian,  and  Orion 
and  Canis  Major  have  set,  or  are  low  down  in  the 
southwest. 

In  mid-heaven,  southeast  of  the  zenith,  is  Arcturus,  cf 
a    dingy    yellow    col- 
our,  but   one   of  the 
brightest  first  magni- 
tude stars. 

East  of  Arcturus 
(now  below  it)  is 
Corona  Borealis,  the 
Northern  Crown,  a 
beautiful  semicircle  of 
stars,  of  which  the 
brightest  is  of  the 
second  magnitude.  FIG.  58.—  The  NortJiern  Crown. 

Near  the  zenith  is 

Coma  Berenices,  the  Hair  of  Berenice,  a  collection  of 
faint  stars  mostly  of  the  fifth  magnitude.  East  of  south 
across  the  meridian  from  Leo  is  Virgo,  the  Virgin,  con- 
spicuous only  by  Spica,  a  white  star  of  nearly  the  first 
magnitude.  Libra,  the  Balance,  east  and  southeast  of 
Virgo,  has  no  conspicuous  stars. 

The  Summer  Constellations 

The  fourth  position  of  the  sphere,  eighteen  hours 
sidereal  time,  occurs  in  May  at  two  A.  M. ;  in  August  at 
eight  P.  M.  Capella  has  now  set,  Lyra  is  near  the  zenith, 


318 


THE    FIXED    STARS 


FIG.  59. — Aquila. 


Cassiopeia  is  in  the  northeast,  and  the  most  splendid  por- 
tion of  the  Milky  Way  is  near  the  meridian.  We  have 
described  all  the  constellations 
that  lie  near  its  course  north  of 
Lyra;  let  us  now  trace  it  to  the 
south. 

One  of  the  noticeable  fea- 
tures of  the  Milky  Way  now  to 
be  seen  is  the  great  bifurcation 
or  separation  into  two  branches. 
The  split  can  be  traced  from 
Cygnus,  where  it  begins,  past 
Lyra  and  halfway  to  the  south- 
ern horizon.  Here  we  see  Aquila, 
the  Eagle,  in  the  cleft,  marked  by  Altair,  of  the  first 
magnitude.  It  is  in  a  line  between  two  other  stars  of  the 
third  and  fourth  magnitudes. 

At  this  point  the  westernmost  branch  of  the  Milky] 
Wray    diverges    yet     farther    and 
seems  to  terminate,  but  if  the  air 
is  clear  we  shall  see  that  it  recom- 
mences near  the  horizon. 

East  of  Aquila  is  a  small  but 
very  pretty  constellation  of  which 
the  scientific  name  is  Delphinus, 
the  Dolphin,  but  which  is  popu- 
larly known  as  Job's  Coffin. 

Between  Lyra  and  the  beautiful 
Corona,  now  some  distance  west  of 


the  zenith,  lies  the  widely  extended 


FIG.  60. — Delphinus,  the 
Dolphin. 


THE  SUMMER  CONSTELLATIONS       319 


Fio.  61, — The  Great  Cluster  of  Hercules,  photographed  at  the  Lick  Observatory 


320 


THE    FIXED    STARS 


V 


constellation,  Hercules.  Alpha,  its  brightest  star,  is  below 
the  second  magnitude  and  may  be  known  by  its  reddish 
colour  and  by  a  white  star,  Alpha  Ophiuchi,  a  little 
farther  east.  The  most  remarkable  object  in  this  con- 
stellation is  the  Great  Cluster  of  Hercules  which,  to  the 
naked  eye,  is  a  very  faint  patch,  but  which  a  great  tele- 
scope resolves  into  a  universe  of  stars. 

Near  the  horizon,  west  of  south,  is  the  zodiacal  con- 
stellation Scorpius,  the  Scorpion.  Its  western  boundary 

is  a  curved  row  of  stars 
forming  the  claws  of  the 
animal;  east  of  them  is 
Antares,  or  Alpha  Scor- 
pii,  reddish  in  colour, 
and  nearly  of  the  first 
magnitude. 

In  the  Milky  Way, 
due  south,  and  therefore 
east  of  Scorpius,  is  Sag- 
ittarius, the  Archer,  with 
quite  a  collection  of 
stars  of  the  second  and 

third  magnitudes.  The  bow  and  arrow  of  the  archer  are 
easily  imagined. 

Next  toward  the  east  are  Capricornus  and  Aquarius, 
already  mentioned.  The  brightest  star  in  the  former 
has  a  companion  so  close  to  it  that  it  is  a  sign  of  not  bad 
eyesight  to  be  able  to  distinguish  it. 


FIG.  62.  — 


the  Scorpion. 


IV 

THE  DISTANCES  OF  THE  STAKS 

THE  principles  on  which  distances  in  the  heavens  are 
determined  was  set  forth  in  our  chapter  explaining  how 
the  heavens  are  measured.  For  distances  of  the  moon  and 
nearer  planets,  we  use,  as  a  base  line  for  measurement,  the 
radius  of  the  earth,  or  the  line  joining  two  points  of  ob- 
servation on  its  surface.  But  this  is  entirely  too  short  to 
serve  for  measuring  a  distance  so  great  as  that  even  of  the 
nearest  star.  For  this  purpose  we  take  as  a  base  line  the 
whole  diameter  of  the  earth's  orbit.  As  the  earth  moves 
from  one  side  of  the  orbit  to  the  other,  the  stars  must  seem 
to  have  a  slight  motion  in  the  opposite  direction.  But  this 
motion  is  found  to  be  almost  immeasurably  small.  It  can 
be  made  out  with  sufficient  precision  only  by  comparing 
the  stars  among  themselves  in  the  following  way : 

Let  the  little  circle  on  the  left  of  the  following  figure 
represent  the  orbit  of  the  earth.  Let  S  be  the  star,  sup- 
posed to  be  near  us,  of  which  we  wish  to  measure  the  dis- 
tance. Let  the  dotted  lines  almost  parallel  to  each  other 
show  the  direction  of  a  star  T  many  times  farther  away. 
When  the  earth  is  at  one  side  of  its  orbit,  say  at  P,  we 
measure  the  small  angle  SPT,  which  seems  to  us  to  sepa- 
rate these  two  stars.  When  the  earth  goes  to  the  opposite 
side,  it  is  readily  seen  that  the  corresponding  angle  SQT 
will  be  greater.  We  again  measure  it.  The  difference 


322  THE    FIXED    STARS 

between  these  two  angles  will  furnish  a  basis  for  com- 
puting, by  trigonometric  methods,  the  distance  of  the 
nearest  star  when  that  of  the  farthest  is  known.  Practi- 
cally we  have  to  assume  that  the  star  T  is  at  an  infinite  dis- 
tance, so  that  the  dotted  lines  are  parallel.  Then  the 
measured  difference  between  the  angles  will  enable  us  to 
calculate  the  angle  subtended  by  the  radius  of  the  earth's 
orbit,  as  seen  from  the  star  S.  This  angle  is  what  astrono- 


FIG.  63. — Measurement  of  the  Parallax  of  a  Star. 

mers  habitually  use  in  their  computations,  not  the  dis- 
tance of  the  star.  It  is  called  the  Parallax  of  the  star. 
If  we  wish  to  obtain  the  distance  of  the  star,  we  have  to 
divide  the  number  206,265  by  the  parallax  of  the  star 
expressed  as  a  fraction  of  a  second.  This  will  give  its 
distance  in  terms  of  the  radius  of  the  earth's  orbit  as  a 
unit  of  measure.  One  second  is  the  angle  subtended  by 
an  object  one  inch  in  diameter  at  a  distance  of  206,265 
inches,  or  more  than  three  miles.  It  is,  of  course,  com- 
pletely invisible  to  the  naked  eye. 

It  will  be  seen  that  this  method  of  measurement  implies 
that  we  know  which  of  the  two  stars  is  the  nearer ;  in  fact, 
that  we  know  the  farther  star  to  be  at  an  almost  infinite 
distance.  The  question  may  be  asked  how  this  knowl- 
edge is  obtained,  and  how  a  star  is  selected  as  being  near 
to  us.  The  most  careful  measures  that  can  be  made  with 
the  finest  instruments  show  that  the  great  mass  of  small 


THE    DISTANCES    OF    THE    STARS     323 

telescopic  stars  do  not  have  the  slightest  change  in  their 
relative  positions,  but  remain  as  if  fixed  on  the  celestial 
sphere  from  year  to  year.  Now  and  then,  however,  an 
exception  is  found.  A  very  bright  star  is  probably  nearer 
to  us  than  the  fainter  ones,  and  if  a  star  shows  any 
change  in  its  position,  the  astronomer  may  proceed  to 
measure  and  determine  its  parallax. 

So  far  as  has  yet  been  determined,  the  nearest  star  to 
us  is  Alpha  Centauri,  a  star  of  nearly  the  first  magnitude, 
in  the  southern  hemisphere.  The  parallax  of  this  star 
is  0.75".  By  the  rule  we  have  given,  its  distance  will  be 
nearly  275,000  times  that  of  the  sun.  Such  a  distance 
transcends  all  our  power  of  conception  over  and  over 
again.  A  crude  idea  of  it  may  be  obtained  by  reflecting 
that  light  itself,  the  speed  of  which  we  have  already 
described,  would  require  more  than  four  years  to  reach 
us  from  this  star.  We  see  the  latter,  not  as  it  is  now,  but 
as  it  was  more  than  four  years  ago.  At  such  a  distance 
not  only  does  the  earth's  orbit  itself  vanish  away  to  a 
point,  but  a  ball  as  large  as  the  whole  body  of  Neptune 
would  be  barely  visible  to  the  naked  eye  as  the  minutest 
possible  point. 

The  next  star  in  the  order  of  distance  is  supposed  to  be 
about  one  half  as  far  again  as  Alpha  Centauri,  and  there 
are  some  half  dozen  others,  within  three  or  four  times  its 
distance.  In  all,  the  parallaxes  of  about  one  hundred 
stars  have  been  determined  with  more  or  less  exactness; 
but  even  in  these  cases  the  parallax  is  sometimes  so  small 
that  we  cannot  be  sure  it  is  real.  It  seems  likely  that  only 
about  fifty  stars  are  within  seven  times  the  distance  of 


324  THE    FIXED    STARS 

Alpha  Centauri.  The  distance  of  the  stars  whose  paral- 
laxes are  too  small  to  be  measured  is  a  matter  of  judg- 
ment rather  than  calculation.  The  probability  seems  to 
be  that  at  least  the  brighter  stars  are  scattered  through 
space  with  some  approach  to  uniformity.  If  this  is  the 
case,  many  of  the  fainter  telescopic  stars,  perhaps  the 
large  majority  of  the  smallest  ones  found  on  photo- 
graphs of  the  heavens,  must  be  more  than  one  thousand 
times  the  distance  of  Alpha  Centauri.  The  light  by 
which  their  presence  is  made  known  to  us  must  have  been 
on  its  way  to  our  system  during  the  whole  period  of 
human  history. 


V 

THE   MOTIONS  OF   THE   STARS 

IF  I  were  asked  what  is  the  greatest  fact  that  the  intel- 
lect of  man  has  ever  brought  to  light  I  should  say  it  was 
this: 

Through  all  human  history,  nay,  so  far  as  we  can  dis- 
cover, from  the  infancy  of  time,  our  solar  system — sun, 
planets,  and  moons — has  been  flying  through  space 
toward  the  constellation  Lyra  with  a  speed  of  which  we 
have  no  example  on  earth.  To  form  a  conception  of  this 
fact  the  reader  has  only  to  look  at  the  beautiful  Lyra  and 
reflect  that  for  every  second  that  the  clock  tells  off,  we 
are  ten  miles  nearer  to  that  constellation.  Every  day  that 
we  live  we  are  nearer  to  it  by  almost,  perhaps  quite,  a 
million  of  miles.  For  every  sentence  that  we  utter,  for 
every  step  that  we  take  in  the  streets  we  are  miles  nearer 
to  this  star.  We  approached  it  by  tens  of  thousands  of 
miles  while  the  writer  has  been  penning  these  lines,  and 
the  reader  has  been  carried  nearer  by  a  thousand  miles 
while  perusing  them.  This  has  been  going  on  through 
all  human  history,  and  we  have  reason  to  believe  that  it 
will  remain  true  for  our  remotest  posterity.  One  of  the 
greatest  problems  of  astronomy  is,  when  and  how  did 
this  journey  begin  and  when  and  how  will  it  end?  Before 
this  question  our  science  stands  dumb.  The  astronomer 
can  tell  no  more  about  the  beginning  or  the  end  of  the 


326  THE    FIXED    STARS 

journey  than  can  the  untutored  child.  He  can  only  im- 
press upon  the  mind  of  his  followers  the  magnitude  of 
the  problem. 

Nothing  can  give  us  a  better  conception  of  the  enor- 
mous distance  of  the  stars  than  the  reflection  that  not- 
withstanding the  rapid  motion,  carrying  us  unceasingly 
forward  through  all  the  ages  that  the  human  race  has 
existed  on  earth,  ordinary  observation  would  fail  to  show 
any  change  in  the  appearance  of  the  constellation  toward 
which  we  are  travelling.  From  what  we  know  of  the  dis- 
tance of  Vega  we  have  reason  to  suppose  that  our  solar 
system  will  not  reach  the  region  in  which  that  star  is  now 
situated  until  the  end  of  a  period  ranging  somewhere 
between  half  a  million  and  a  million  of  years  from  the 
present  time. 

It  does  not  follow,  however,  that  our  posterity,  if  any 
such  shall  then  live  on  the  earth,  will  find  Vega  when  they 
arrive  at  its  present  place.  It  also  is  going  on  its  own 
journey  and  is  passing  away  from  its  present  location 
almost  as  rapidly  as  we  are  approaching  it. 

What  is  true  of  our  sun  and  of  Vega  is  true,  so  far  as 
we  know,  of  every  star  in  the  heavens.  Each  of  these 
bodies  is  flying  straight  ahead  through  space  like  a  ball 
shot  out  from  a  cannon,  with  a  speed  which  in  most  cases 
is  almost  inconceivable.  It  would  be  a  very  slow  moving 
star  of  which  the  velocity  did  not  exceed  that  of  a  cannon 
shot.  In  the  great  majority  of  cases  it  ranges  from  five 
to  thirty  miles  per  second — frequently  more  than  fifty 
miles.  Indeed  there  are  two  stars,  of  which  Arcturus  is 
one,  whose  speed  we  have  reason  to  believe  approaches 


THE    MOTIONS    OF    THE    STARS       327 

two  hundred  miles  a  second.  These  motions  of  the  stars 
are  called  their  proper  motions. 

We  have  described  the  proper  motions  as  so  many  miles 
per  second.  But  owing  to  the  enormous  distance  of  the 
stars,  rapid  as  the  proper  motions  are  in  reality,  they 
seem  slow  indeed  when  we  observe  them.  So  slow  are  they 
that  if  Ptolemy  should  come  to  life  after  his  sleep  of  near- 
ly eighteen  hundred  years,  and  be  asked  to  compare  the 
heavens  as  they  are  now  with  those  of  his  time,  he  would 
not  be  able  to  see  the  slightest  difference  in  the  configura- 
tion of  a  single  constellation.  Even  to  the  oldest  Assyrian 
priests,  the  constellation  Lyra  and  the  star  Vega  looked 
exactly  as  they  do  to  us  to-day,  notwithstanding  the  im- 
measurable distance  by  which  we  have  approached  them. 

To  resuscitate  an  inhabitant  of  .the  ancient  world  who 
would  be  able  to  perceive  any  change,  we  should  have  to 
go  back  four  thousand  years  perhaps,  to  the  time  of  Job, 
and  we  should  have  to  take  one  of  the  swiftest  moving 
stars  in  the  heavens,  Arcturus.  Bringing  Job  to  life  and 
showing  him  the  constellation  Bootes,  of  which  Arcturus 
is  the  brightest  star,  he  would  perceive  the  latter  to  have 
moved  through  about  half  of  the  distance  in  the  accom- 
panying diagram  between  the  stars  marked  "1"  and  "52." 

In  considering  these  motions,  the  most  natural  thought 
to  present  itself  is  that  the  stars  are  describing  vastly  ex- 
tended orbits  around  some  centre,  as  the  planets  are 
moving  round  the  sun,  and  that  the  motions  we  see  are 
simply  the  motions  in  these  orbits.  But  the  facts  do  not 
support  this  view.  The  most  refined  observations  yet 
made  do  not  show  the  slightest  curvature  in  the  path  of 


THE    FIXED    STARS 

* 

any  star.  Every  one  seems  to  be  going  straight  ahead  on 
its  own  account,  never  swerving  to  the  right  or  left.  It 
does  not  seem  possible  to  admit  the  existence  of  bodies 
large  and  massive  enough  to  control  such  rapid  motions. 
A  body  massive  enough  to  attract  Arcturus  from  its  head- 


PiG.  64. — Arcturus  and  the  Surrounding  Stars  in  Constellation  Bootes. 

long  course  would  throw  all  that  part  of  the  universe  in 
which  we  live  into  disorder.  The  problem  where  the 
rapidly  moving  stars  came  from  and  whither  they  are 
going  is  therefore  for  us  insoluble.  What  makes  the  case 
yet  more  difficult  is  that  different  stars  move  in  different 
directions,  without  any  seeming  order,  so  that  one  motion 
seems  to  have  no  connection  with  another,  unless  in  a  few 
very  rare  cases. 


VI 

VARIABLE    AND    COMPOUND    STARS 

As  a  general  rule  the  starry  heavens  may  be  taken  as 
a  symbol  of  eternal  unchangeability.  The  proverb- 
makers  have  told  us  in  all  time  how  everything  on  the 
earth  is  subject  to  alternation  and  decay,  while  the  stars 
of  heaven  remain  as  we  see  them,  age  after  age.  But  it 
is  now  known  that,  although  this  is  true  of  the  great 
majority  of  the  stars,  there  are  some  exceptions.  These 
are  so  little  striking  that  they  were  never  noticed  by  the 
ancient  astronomers. 

The  first  person  in  history  to  observe  a  change  in  a  star 
was  one  Daniel  Fabritius,  a  diligent  watcher  of  the 
heavens,  who  lived  three  centuries  ago. 

In  August,  1596,  he  noticed  a  star  of  the  third  magni- 
tude before  unknown  in  the  constellation  Cetus,  which 
soon  faded  away  again,  and  disappeared  from  view  in 
October.  In  subsequent  years  it  was  found  to  show 
itself  at  regular  intervals  of  about  eleven  months. 

Two  centuries  elapsed  before  another  case  of  the  kind 
was  known.  Then  it  was  found  that  the  star  Algol,  in 
Perseus,  faded  away  from  the  second  to  the  fourth  magni- 
tude for  a  few  hours  at  intervals  of  a  little  less  than  three 
days. 

Early  in  the  nineteenth  century  other  stars  were  found 
to  be  subject  to  a  more  or  less  regular  variation  of  their 


330  THE    FIXED    STARS 

light.  As  observers  studied  the  heavens  with  greater  care, 
more  and  more  of  such  stars  were  found,  until  at  the  pres- 
ent time  the  list  of  them  numbers  four  or  five  hundred, 
and  is  constantly  increasing.  Of  these  some  vary  in  an 
irregular  way,  but  a  large  majority  go  through  a  regu- 
lar period. 

The  easiest  of  these  objects  to  notice  is  Beta  Lyra, 
which  is  marked  B  on  the  figure  of  that  constellation 
already  given.  It  can  be  seen  at  some  hour  of  any  clear 
evening,  spring,  summer,  or  autumn.  If  the  reader  as  he 
takes  his  evening  walk  will,  night  after  night,  compare 
this  star  with  the  one  nearest  to  it  and  nearly  of  the  same 
magnitude,  he  will  see  that  while  on  some  evenings  the  two 
appear  perfectly  equal,  on  others  Beta  will  be  of  a  mag- 
nitude fainter  than  the  other.  Careful  and  continued 
watching  will  show  that  the  change  takes  place  in  a 
period  of  about  six  days  and  a  half.  That  is  to  say,  if 
the  two  stars  are  equal  on  a  certain  evening,  they  will 
again  appear  equal  at  the  end  of  six  or  seven  days,  and 
so  on  indefinitely.  Midway  between  the  two  times  of 
equality  the  variable  one  will  be  at  its  faintest.  If  the 
observer  notes  the  magnitudes  at  this  time  with  the 
greatest  precision,  a  curious  fact  will  be  brought  out. 
Every  alternate  minimum,  as  the  phase  of  least  light  is 
called,  is  slightly  fainter  than  that  preceding  or  follow- 
ing. The  actual  period  is  therefore  nearly  thirteen  days, 
during  which  time  there  are  two  maxima  of  equal  bright- 
ness.and  two  slightly  different  minima. 

It  is  now  known  that  the  variation  of  light  in  this  case 
is  not  really  inherent  in  the  star  itself,  but  arises  from 


VARIABLE  AND  COMPOUND  STARS     331 

the  fact  that  the  star  is  a  double  one,  composed  of  two 
stars  revolving  around  each  other,  and  so  near  together 
as  almost  to  touch.  As  they  revolve,  each  one  in  succes- 
sion wholly  or  partially  hides  the  other.  This  fact  is  not 
brought  out  by  the  telescope,  because  the  most  powerful 
telescope  that  could  be  made  would  not  show  the  two 
stars  separately.  It  is  the  result  of  long  and  careful 
study  of  the  spectrum  of  the  star,  which  is  found  to  be 
a  double  one,  the  lines  in  one  of  which  alternately  cover 
and  recede  from  the  lines  of  the  other. 

In  the  extent  of  variation  of  its  light  the  most  remark- 
able of  the  more  conspicuous  variable  stars  is  Omicron 
Ccti,  already  mentioned  as  seen  by  Fabritius.  It  is  now 
found  to  go  through  a  regular  period  in  three  hundred 
and  thirty  days.  During  about  two  weeks  of  this  time  it 
is  at  its  brightest,  and  is  then  sometimes  of  the  second 
magnitude  and  sometimes  much,  fainter — occasionally 
only  of  the  fifth.  After  each  maximum  it  gradually 
fades  away  for  a  few  weeks  and  disappears  from  view  to 
the  naked  eye.  But  with  a  telescope  it  can  be  seen  all  the 
year  round. 

The  period  of  eleven  months  makes  the  maximum  occur 
about  a  month  earlier  every  year.  During  some  years  it 
will  occur  when  the  star  is  so  near  the  sun  that  it  cannot 
be  easily  observed.  This  will  be  the  case  during  the  years 
1903-'05. 

Algol,  also  called  Beta  Persei,  being  in  northern  dec- 
lination, can  be  seen  in  our  latitudes  at  some  time  on 
almost  every  night  of  the  year.  In  autumn  and  winter 
it  is  visible  in  the  early  evening.  The  peculiarity  of  its 


332  THE    FIXED    STARS 

variation  is  that  it  remains  of  the  same  brightness  nearly 
all  the  time,  but  fades  away  for  a  few  hours  at  intervals 
of  about  two  days  and  twenty-one  hours.  It  is  now 
known  that  this  is  due  to  the  partial  eclipse  of  the  ctar 
by  a  dark  body  nearly  as  large  as  itself,  revolving  round 
it.  It  is  true  that  this  body  has  never  been  seen  by  human 
eye  and  never  will  be.  Its  existence  is  made  known  by  its 
causing  the  star  to  revolve  in  a  small  orbit.  It  is  true 
that  this  motion  of  the  bright  star  is  too  small  to  be 
observed  with  the  telescope,  but  it  is  made  certain  by 
means  of  the  spectroscope,  which  shows  a  change  in  the 
wave  length  of  the  light  coming  from  the  star. 

Different  variable  stars  differ  very  widely  in  the  extent 
of  their  variation.  In  most  cases  the  latter  is  so  slight 
that  only  an  expert  observer  would  notice  it.  Frequently 
it  cannot  be  determined  until  after  a  long  study  by 
various  observers  whether  a  "suspected  variable"  is  really 
such. 

These  objects  form  a  very  interesting  subject  of  ob- 
servation for  those  who  have  at  command  little  or  no 
instrumental  facilities.  No  telescope  is  needed  unless  the 
star  is,  at  some  of  its  phases,  invisible  to  the  naked  eye. 
The  points  to  be  noticed  and  recorded  are  the  exact 
magnitude  of  the  star  from  minute  to  minute  or  hour  to 
hour,  as  it  is  going  through  its  most  rapid  change,  in 
order  to  learn  at  what  moment  its  brightness  is  greatest 
or  least. 

What  adds  to  the  interest  of  the  astronomer  in  these 
objects  is  the  evidence  now  being  gathered  that  many, 
perhaps  most  of  the  stars,  are  not  single  bodies,  but  more 


VARIABLE  AND  COMPOUND  STARS     333 

or  less  complex  systems  of  bodies  having  the  widest  di- 
versity in  their  construction.  Double  stars  have  been 
familiar  to  every  observer  of  the  heavens  since  the  time 
of  the  great  Herschel.  But  it  is  only  in  the  time  of  our 
generation  that  the  spectroscope  has  begun  to  make 
known  to  us  pairs  of  stars  revolving  round  each  other, 
of  which  the  components  are  so  close  together  that  the 
most  powerful  telescope  can  never  separate  them.  The 
history  of  science  offers  no  greater  marvel  than  the  dis- 
coveries of  invisible  planets  moving  round  many  of  the 
stars  which  are  now  being  made,  and  in  which  the  Lick 
observatory  has  recently  taken  the  lead. 

It  now  seems  more  or  less  probable  that  the  changes  of 
light  in  all  stars  having  a  regular  and  constant  period  is 
due  to  the  revolution  of  large  planets  or  other  stars 
around  them.  Sometimes  the  variation  is  slight  and  is 
caused  in  the  way  we  have  described,  by  one  body  par- 
tially eclipsing  the  other  as  it  passes  across  it.  In  this 
case  there  may  be  no  real  variation  in  the  light ;  the  star 
eclipsed  shines  just  as  bright  behind  the  eclipsing  body 
as  when  it  is  not  eclipsed.  But  it  now  seems  that,  if  the 
darker  body  revolves  in  a  very  eccentric  orbit,  so  as  to  be 
much  nearer  the  bright  body  at  some  times  than  at  others, 
its  attraction  produces  such  a  change  in  the  other  as  to 
greatly  increase  its  light.  Just  how  this  effect  is  pro- 
duced it  is  as  yet  impossible  to  say. 


THE  END. 


k 


THE  COUNTRY  LIFE  PRESS 
GARDEN  CITY,  N.  Y. 


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